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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> > @tru I'd ask @AaronBallman. My vote would be to reformat.
> 
> Not as a requirement for this patch, I imagine?

If we wanted to reformat the file as an NFC commit (before or after this patch), that would be fine. But let's please not reformat as part of this patch (that makes code archeology much harder).

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