[clang] [compiler-rt] [llvm] [AIX] Implement the ifunc attribute. (PR #153049)

[Wael Yehia via llvm-commits]

w2yehia wrote:

I rebased on the `!implicit.ref` PR, had to change the `.ref` relocation for functions from `.ref foo[DS]` to `.ref .foo[PR]` because when `foo` is just a declaration with no other references in the code, we currently generate a `.extern .foo[PR]` but not a `.extern foo[DS]` unless `foo` is address taken.
I initially tried to change the query `Function::hasAddressTaken` to return true if the given function is on an `!implicit.ref` metadata, but that turned out to be non-trivial.

I'm still not sure if `Function::hasAddressTaken` should return true if the only reference to a function declaration is a use on a `!implicit.ref` MD.

Sean is reviewing.

https://github.com/llvm/llvm-project/pull/153049