[Verifier] Restore defined-resolver verification for IFuncs

Now that clang no longer emits GlobalIFunc-s with a
declaration for a resolver, we can restore that check.
In addition, add a linkage check like the one we have
on GlobalAlias-es, and a Verifier test for ifuncs.

Signed-off-by: Itay Bookstein <ibookstein@gmail.com>

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D120267