[llvm] [IR] enable attaching metadata on ifuncs (PR #158732)

[Daniel Kiss via llvm-commits]

DanielKristofKiss wrote:

> I used [D21052](https://reviews.llvm.org/D21052) as a reference. [There](https://reviews.llvm.org/D21052#457917) it was suggested that function declarations should not have `!dbg` or `!prof` (though later a unique `!dbg` attachement was allowed). I'm not sure if we have to add anything to the Verifier for metadata on an ifunc.

`!dbg` might be useful but ifunc itself can't be profiled so no need for `!prof`.

LGMT but lets wait to see what others say.

https://github.com/llvm/llvm-project/pull/158732