MS ABI: Use musttail for thunk IR generation

This allows us to perfectly forward non-trivial arguments that use
inalloca.

We still can't forward non-trivial arguments through thunks when we have
a covariant return type with a non-trivial adjustment.  This would
require emitting an extra copy, which is non-conforming anyway.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@208927 91177308-0d34-0410-b5e6-96231b3b80d8