Provide, and document, a set of __c11_atomic_* intrinsics to implement C11's
<stdatomic.h> header.

In passing, fix LanguageExtensions to note that C11 and C++11 are no longer
"upcoming standards" but are now actually standardized.

llvm-svn: 154513
diff --git a/clang/lib/AST/ExprConstant.cpp b/clang/lib/AST/ExprConstant.cpp
index 435d94c1..ce41308 100644
--- a/clang/lib/AST/ExprConstant.cpp
+++ b/clang/lib/AST/ExprConstant.cpp
@@ -4365,7 +4365,8 @@
       
     return Error(E);
 
-  case Builtin::BI__atomic_is_lock_free: {
+  case Builtin::BI__atomic_is_lock_free:
+  case Builtin::BI__c11_atomic_is_lock_free: {
     APSInt SizeVal;
     if (!EvaluateInteger(E->getArg(0), SizeVal, Info))
       return false;