[CIR] Upstream vtable thunk handling (#183629)
This implements vtable thunk handling in CIR based on the incubator
code, but also compared against the latest Clang LLVM IR codegen.
Eventually, we'll want to create CIR abstractions for all of this and
move the CXXABI-specific details into the CXXABI lowering pass. For now,
we just implement it directly in codegen.
