use __thiscall. (This doesn't actually work for MSVC; they don't allow the
__thiscall qualifier here, but it's sufficient to demonstrate that we do
implement the intent of the DR.)


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@217213 91177308-0d34-0410-b5e6-96231b3b80d8