[clang] [CIR] Upstream support for function/call calling conventions (PR #181170)

[Sirui Mu via cfe-commits]

https://github.com/Lancern requested changes to this pull request.

I think this PR is a bit too big for review. It contains multiple calling conventions for multiple targets. Could you simplify this PR and make it include only the C calling convention `cir::CallingConv::C`? Other calling conventions could be introduced in future PRs.

https://github.com/llvm/llvm-project/pull/181170