Compare canonical return types when generating MS C++ ABI vtable thunks

This was part of the cause for PR17655.  We were generating thunks when
we shouldn't have.  I suspect that if we tweak the test case for PR17655
to actually require thunks, we can reproduce the same crash.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@197836 91177308-0d34-0410-b5e6-96231b3b80d8