[llvm] Handle dso_local_equivalent in FunctionComparator

This addresses https://github.com/llvm/llvm-project/issues/51066.

Prior to this, dso_local_equivalent would lead to an llvm_unreachable in
a switch in the FunctionComparator. This adds a conservative case in
that switch that just compares the underlying functions.

Differential Revision: https://reviews.llvm.org/D134300