Bug 1025143 - Fix a compilation error in TestTypedEnum.cpp with clang-cl; r=bjacob

This makes the code more correct since if we don't have constexpr
support in the compiler, the CastableTypedEnumResult type would
actually not be a literal type, so we should not assert that here.

mfbt/tests/TestTypedEnum.cpp

@@ -38,7 +38,7 @@
 #  endif
 #endif
 
-#ifdef MOZ_HAVE_IS_LITERAL
+#if defined(MOZ_HAVE_IS_LITERAL) && defined(MOZ_HAVE_CXX11_CONSTEXPR)
 #include <type_traits>
 template<typename T>
 void