Form a CallExpr from __noop without parens

MSVC accepts __noop without any trailing parens and treats it like a
literal zero.  We don't treat __noop as an integer literal, but now at
least we can parse a naked __noop expression.

Reviewers: rsmith

Differential Revision: http://reviews.llvm.org/D4476

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@212860 91177308-0d34-0410-b5e6-96231b3b80d8