MS extension: Make __noop be the integer zero, not void

We still don't accept '__noop;', and we don't consider __noop to be the
integer literal zero.  More work is needed.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@212839 91177308-0d34-0410-b5e6-96231b3b80d8