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

[Craig Topper via llvm-commits]

topperc wrote:

> 2 only happens when users use __attribute__((target("xxxx"))), but user cannot use a xxxx if it doesn't exist at all. As long as we define a new fastcc implementation together when defining the new feature, there won't be a problem.

It can also happen in LTO if different modules being linked together used different cpu or features on their command lines.

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