Improve experience of debugging `libclang-pathogen`

[PathogenDavid]

Semi-inspired by investigations for MochiLibraries/Mochi.PhysX#5

• Publish a debug variant of the native runtime
• Enable source indexing
• Investigate enabling PDBs even on release builds for Windows.
• Put PDBs and DLLs on a symbol server (see Figure out how to distribute native PDBs Mochi.PhysX#5)
• Look into doing equivalent for Linux builds using debuginfod (hosting a server is slightly more complex here. Could be a static file server but canonical way is to use their HTTP daemon. Server is expected to provide source. See man page.)

Judging by this comment thread on Hacker News, I don't think there's really any equivalent for macOS.