[llvm] Teach whole program devirtualization about relative vtables

Prior to this patch, WPD was not acting on relative-vtables in C++. This
involves teaching WPD about these things:

- llvm.load.relative which is how relative-vtables are indexed (instead of GEP)
- dso_local_equivalent which is used in the vtable itself when taking the
  offset between a virtual function and vtable
- Update llvm/test/ThinLTO/X86/devirt.ll to use opaque pointers and add
  equivalent tests for RV

Differential Revision: https://reviews.llvm.org/D134320