Only provide MS builtins when -fms-extensions is on

We already have builtins that are only available in GNU mode, so this
mirrors that.

Reviewers: rsmith

Differential Revision: http://llvm-reviews.chandlerc.com/D2128

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@194615 91177308-0d34-0410-b5e6-96231b3b80d8