MS ABI: The latest VC "14" CTP implements deleted virtual functions

Deleted virtual functions get _purecall inserted into the vftable.
Earlier CTPs would simply stick nullptr in there.

N.B.  MSVC can't handle deleted virtual functions which require return
adjusting thunks, they give an error that a deleted function couldn't be
called inside of a compiler generated function.  We get this correct by
making the thunk have a __purecall entry as well.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@217654 91177308-0d34-0410-b5e6-96231b3b80d8