[Clang] [Sema] Ignore invalid multiversion function redeclarations

If a redeclaration of a multiversion function is invalid,
it may be in a broken condition (for example, missing an important
attribute). We shouldn't analyze invalid redeclarations.

Fixes https://github.com/llvm/llvm-project/issues/57343

Reviewed By: tahonermann

Differential Revision: https://reviews.llvm.org/D133641