[ms-cxxabi] Emit fewer trivial return adjusting thunks

Most importantly, this makes our vtable layout match MSVC's.  Previously
we would emit a return adjusting thunk whenever the return types
differed, even if the adjustment would have been trivial.

MSVC does emit some trivial return adjusting thunks, but only if there
was already an overridden method that required a return adjustment.

llvm-svn: 198080