[LTO][LLD] Prevent invalid LTO libfunc transforms

[mysterymath]

This patch ensures that:

1. New bitcode is not extracted for libfuncs after LTO occurs, and
2. Extracted bitcode for libfuncs is considered external, since new
   calls to it may be emitted.

[github-actions]

✅ With the latest revision this PR passed the C/C++ code formatter.

[github-actions]

🐧 Linux x64 Test Results

• 84392 tests passed
• 1730 tests skipped

All tests passed but another part of the build failed. Click on a failure below to see the details.

tools/clang/lib/CodeGen/CMakeFiles/obj.clangCodeGen.dir/BackendUtil.cpp.o

FAILED: tools/clang/lib/CodeGen/CMakeFiles/obj.clangCodeGen.dir/BackendUtil.cpp.o
sccache /opt/llvm/bin/clang++ -DCLANG_EXPORTS -DGTEST_HAS_RTTI=0 -D_DEBUG -D_GLIBCXX_ASSERTIONS -D_GLIBCXX_USE_CXX11_ABI=1 -D_GNU_SOURCE -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -I/home/gha/actions-runner/_work/llvm-project/llvm-project/build/tools/clang/lib/CodeGen -I/home/gha/actions-runner/_work/llvm-project/llvm-project/clang/lib/CodeGen -I/home/gha/actions-runner/_work/llvm-project/llvm-project/clang/include -I/home/gha/actions-runner/_work/llvm-project/llvm-project/build/tools/clang/include -I/home/gha/actions-runner/_work/llvm-project/llvm-project/build/include -I/home/gha/actions-runner/_work/llvm-project/llvm-project/llvm/include -gmlt -fPIC -fno-semantic-interposition -fvisibility-inlines-hidden -Werror -Werror=date-time -Werror=unguarded-availability-new -Wall -Wextra -Wno-unused-parameter -Wwrite-strings -Wcast-qual -Wmissing-field-initializers -pedantic -Wno-long-long -Wc++98-compat-extra-semi -Wimplicit-fallthrough -Wcovered-switch-default -Wno-noexcept-type -Wnon-virtual-dtor -Wdelete-non-virtual-dtor -Wsuggest-override -Wstring-conversion -Wno-pass-failed -Wmisleading-indentation -Wctad-maybe-unsupported -fdiagnostics-color -ffunction-sections -fdata-sections -fno-common -Woverloaded-virtual -Wno-nested-anon-types -O3 -DNDEBUG -std=c++17  -fno-exceptions -funwind-tables -fno-rtti -UNDEBUG -MD -MT tools/clang/lib/CodeGen/CMakeFiles/obj.clangCodeGen.dir/BackendUtil.cpp.o -MF tools/clang/lib/CodeGen/CMakeFiles/obj.clangCodeGen.dir/BackendUtil.cpp.o.d -o tools/clang/lib/CodeGen/CMakeFiles/obj.clangCodeGen.dir/BackendUtil.cpp.o -c /home/gha/actions-runner/_work/llvm-project/llvm-project/clang/lib/CodeGen/BackendUtil.cpp
/home/gha/actions-runner/_work/llvm-project/llvm-project/clang/lib/CodeGen/BackendUtil.cpp:1446:11: error: no matching function for call to 'thinBackend'
1446 |           thinBackend(Conf, -1, AddStream, *M, *CombinedIndex, ImportList,
|           ^~~~~~~~~~~
/home/gha/actions-runner/_work/llvm-project/llvm-project/llvm/include/llvm/LTO/LTOBackend.h:62:1: note: candidate function not viable: no known conversion from 'std::nullptr_t' to 'const SmallVector<StringRef>' for 10th argument
62 | thinBackend(const Config &C, unsigned Task, AddStreamFn AddStream, Module &M,
| ^
63 |             const ModuleSummaryIndex &CombinedIndex,
64 |             const FunctionImporter::ImportMapTy &ImportList,
65 |             const GVSummaryMapTy &DefinedGlobals,
66 |             MapVector<StringRef, BitcodeModule> *ModuleMap, bool CodeGenOnly,
67 |             const SmallVector<StringRef> &BitcodeLibFuncs,
|             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1 error generated.

If these failures are unrelated to your changes (for example tests are broken or flaky at HEAD), please open an issue at https://github.com/llvm/llvm-project/issues and add the infrastructure label.

[github-actions]

🪟 Windows x64 Test Results

• 80010 tests passed
• 1977 tests skipped

All tests passed but another part of the build failed. Click on a failure below to see the details.

[code=1] tools/clang/lib/CodeGen/CMakeFiles/obj.clangCodeGen.dir/BackendUtil.cpp.obj

FAILED: [code=1] tools/clang/lib/CodeGen/CMakeFiles/obj.clangCodeGen.dir/BackendUtil.cpp.obj
sccache C:\clang\clang-msvc\bin\clang-cl.exe  /nologo -TP -DCLANG_BUILD_STATIC -DGTEST_HAS_RTTI=0 -DUNICODE -D_CRT_NONSTDC_NO_DEPRECATE -D_CRT_NONSTDC_NO_WARNINGS -D_CRT_SECURE_NO_DEPRECATE -D_CRT_SECURE_NO_WARNINGS -D_GLIBCXX_ASSERTIONS -D_HAS_EXCEPTIONS=0 -D_SCL_SECURE_NO_DEPRECATE -D_SCL_SECURE_NO_WARNINGS -D_UNICODE -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -Itools\clang\lib\CodeGen -IC:\_work\llvm-project\llvm-project\clang\lib\CodeGen -IC:\_work\llvm-project\llvm-project\clang\include -Itools\clang\include -Iinclude -IC:\_work\llvm-project\llvm-project\llvm\include /DWIN32 /D_WINDOWS   /Zc:inline /Zc:__cplusplus /Oi /Brepro /bigobj /permissive- -Werror=unguarded-availability-new /W4  -Wextra -Wno-unused-parameter -Wwrite-strings -Wcast-qual -Wmissing-field-initializers -Wimplicit-fallthrough -Wcovered-switch-default -Wno-noexcept-type -Wnon-virtual-dtor -Wdelete-non-virtual-dtor -Wsuggest-override -Wstring-conversion -Wno-pass-failed -Wmisleading-indentation -Wctad-maybe-unsupported /Gw /O2 /Ob2  -MD  /EHs-c- /GR- -UNDEBUG -std:c++17 /showIncludes /Fotools\clang\lib\CodeGen\CMakeFiles\obj.clangCodeGen.dir\BackendUtil.cpp.obj /Fdtools\clang\lib\CodeGen\CMakeFiles\obj.clangCodeGen.dir\ -c -- C:\_work\llvm-project\llvm-project\clang\lib\CodeGen\BackendUtil.cpp
C:\_work\llvm-project\llvm-project\clang\lib\CodeGen\BackendUtil.cpp(1446,11): error: no matching function for call to 'thinBackend'
1446 |           thinBackend(Conf, -1, AddStream, *M, *CombinedIndex, ImportList,
|           ^~~~~~~~~~~
C:\_work\llvm-project\llvm-project\llvm\include\llvm/LTO/LTOBackend.h(62,1): note: candidate function not viable: no known conversion from 'std::nullptr_t' to 'const SmallVector<StringRef>' for 10th argument
62 | thinBackend(const Config &C, unsigned Task, AddStreamFn AddStream, Module &M,
| ^
63 |             const ModuleSummaryIndex &CombinedIndex,
64 |             const FunctionImporter::ImportMapTy &ImportList,
65 |             const GVSummaryMapTy &DefinedGlobals,
66 |             MapVector<StringRef, BitcodeModule> *ModuleMap, bool CodeGenOnly,
67 |             const SmallVector<StringRef> &BitcodeLibFuncs,
|             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1 error generated.

If these failures are unrelated to your changes (for example tests are broken or flaky at HEAD), please open an issue at https://github.com/llvm/llvm-project/issues and add the infrastructure label.