[clang] [clang][bytecode] Reject all function calls in C (PR #175920)

[Timm Baeder via cfe-commits]

tbaederr wrote:

> > C doesn't have constexpr functions, so this can't ever work anyway.
> > Fixes #175877
> 
> This isn't quite true because of builtins: https://godbolt.org/z/j5oYen17r does that still work after these changes?

Yes, builtins go throug `CallBI` instead.

https://github.com/llvm/llvm-project/pull/175920