[clang] [llvm] [GlobalOpt] Add TTI interface useFastCCForInternalCall for FASTCC (PR #164768)

[Alex Bezzubikov via llvm-commits]

zuban32 wrote:

>> Autoupgrade. If you read bitcode generated by an old version of LLVM, does it still have the same meaning?
> I think 1&3 are not real problems. fastcc are internal only calling convention within a module.
I think technically it's still possible to have a problem if we later change the definition of the CC we're substituting `fastcc` with, and try to link with the older object file. That means the implementation of `fastcc` for that particular feature we're setting now should be set once and for all, never to be touched later.

https://github.com/llvm/llvm-project/pull/164768