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

llvm-svn: 194615
diff --git a/clang/test/SemaCXX/builtins.cpp b/clang/test/SemaCXX/builtins.cpp
index 63aa711..69bdfa6 100644
--- a/clang/test/SemaCXX/builtins.cpp
+++ b/clang/test/SemaCXX/builtins.cpp
@@ -38,3 +38,9 @@
 
   S *ptmp = __builtin_addressof(S{}); // expected-error {{taking the address of a temporary}}
 }
+
+void no_ms_builtins() {
+  __assume(1); // expected-error {{use of undeclared}}
+  __noop(1); // expected-error {{use of undeclared}}
+  __debugbreak(); // expected-error {{use of undeclared}}
+}