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/Sema/builtins.c b/clang/test/Sema/builtins.c
index 33b0954..8ca33c8 100644
--- a/clang/test/Sema/builtins.c
+++ b/clang/test/Sema/builtins.c
@@ -191,3 +191,9 @@
   ptr = __builtin___strlcpy_chk(dst, src, sizeof(src), sizeof(dst)); // expected-warning {{incompatible integer to pointer conversion}}
   ptr = __builtin___strlcat_chk(dst, src, sizeof(src), sizeof(dst)); // expected-warning {{incompatible integer to pointer conversion}}
 }
+
+void no_ms_builtins() {
+  __assume(1); // expected-warning {{implicit declaration}}
+  __noop(1); // expected-warning {{implicit declaration}}
+  __debugbreak(); // expected-warning {{implicit declaration}}
+}