[clang] [llvm][tblgen] Add `SourcePath` for `emitSourceFileHeader` (PR #65744)

[Shao-Ce SUN via cfe-commits]

sunshaoce wrote:

I am not sure how to keep only the `llvm-project/xxxx/Filename` path except by hard-coding. Or how to keep only the base file name? This could limit it to within 80 characters.

https://github.com/llvm/llvm-project/pull/65744