MS ABI: Use musttail for vtable thunks that pass arguments by value

This moves some memptr specific code into the generic thunk emission
codepath.

Fixes PR20053.

Reviewers: majnemer

Differential Revision: http://reviews.llvm.org/D4613

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@214004 91177308-0d34-0410-b5e6-96231b3b80d8