[clang] [clang][FMV] Direct-call FMV callees from FMV callers (PR #80093)

[Erich Keane via cfe-commits]

erichkeane wrote:

> My gut feel was that recovering this information from the callee's resolver's body would take heroics if we tried to do it in the backend.

Opt can already see the feature strings in the llvm-attributes, and can introspect into it for the resolver.  I could PERHAPS see value in an llvm-attribute on the resolver to tell OPT to try to look through that (that is, something that says "this is a generated Function MultiVersion resolver, you can trust these conditions match the functions"), but the rest I don't think needs to be in the CFE.

https://github.com/llvm/llvm-project/pull/80093