[clang] [clang][bytecode] Use tailcalls via `[[clang::musttail]]` (PR #173756)

[Martin Storsjö via cfe-commits]

mstorsjo wrote:

> Why does the `has_cpp_attribute` check work if the calling convention is not supported? Anyway, I incorporated that into #188419, thanks.

I guess the issue is that the compiler (frontend/parser) does recognize the attribute and hence marks it as supported, but the actual codegen part knows that it doesn't do anything for this particular target, and thus warns about it.

https://github.com/llvm/llvm-project/pull/173756