add an additional test case for generic attributes

gcc treats [[gnu:const]], [[gnu::__const]], and [[gnu:__const__]] as all being
equivalent.  Add an additional test case to ensure that we do not miss the last
case.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@195982 91177308-0d34-0410-b5e6-96231b3b80d8

test/Parser/cxx0x-attributes.cpp

@@ -309,8 +309,10 @@ namespace GccConst {
   // GCC's tokenizer treats const and __const as the same token.
   [[gnu::const]] int *f1();
   [[gnu::__const]] int *f2();
+  [[gnu::__const__]] int *f3();
   void f(const int *);
   void g() { f(f1()); f(f2()); }
+  void h() { f(f3()); }
 }
 
 namespace GccASan {