[clang] [clang] Use TargetInfo to decide Mangling for C (PR #129920)

[Reid Kleckner via cfe-commits]

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

Thanks, I spent a while thinking if this was correct, and then convincing myself that we ignore the relevant calling conventions on the appropriate platforms (we ignore `__fastcall` on x64 and ignore `__vectorcall` on Win ARM).

https://github.com/llvm/llvm-project/pull/129920