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

[Aaron Ballman via llvm-commits]

https://github.com/AaronBallman approved this pull request.

LGTM (fine to land without tests as it only adds comments to generated header files)

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