Move linker warning/error reporting to their own files

This is a large amount of code that is solely concerned with producing
information that seeks to be helpful to a (human) user. Moving this to
its own translation unit should increase the readability of core linking
algorithms.

Author: tautschnig

src/linking/Makefile

@@ -1,5 +1,6 @@
 SRC = casting_replace_symbol.cpp \
       linking.cpp \
+      linking_diagnostics.cpp \
       remove_internal_symbols.cpp \
       static_lifetime_init.cpp \
       # Empty last line

src/linking/casting_replace_symbol.cpp

@@ -11,6 +11,10 @@ Author: Michael Tautschnig
 
 #include "casting_replace_symbol.h"
 
+#include <util/pointer_expr.h>
+#include <util/std_code.h>
+#include <util/std_expr.h>
+
 bool casting_replace_symbolt::replace(exprt &dest) const
 {
   bool result = true; // unchanged