[YAMLIO] Wrap explicit specialization in llvm::yaml to appease build bots

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@365876 91177308-0d34-0410-b5e6-96231b3b80d8

unittests/Support/YAMLIOTest.cpp

@@ -2627,11 +2627,15 @@ struct FooBarMapMap {
   std::map<std::string, FooBar> fbm;
 };
 
+namespace llvm {
+namespace yaml {
 template <> struct MappingTraits<FooBarMapMap> {
   static void mapping(IO &io, FooBarMapMap &x) {
     io.mapRequired("fbm", x.fbm);
   }
 };
+}
+}
 
 TEST(YAMLIO, TestEmptyMapWrite) {
   FooBarMapMap cont;