[test] Test changes to accommodate LWG 2904 "Make variant move-assignment more exception safe"

Also: Move constexpr / triviality extension tests into the std tree and make them conditional on _LIBCPP_VERSION / _MSVC_STL_VERSION.

https://reviews.llvm.org/D32671

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@304847 91177308-0d34-0410-b5e6-96231b3b80d8

test/support/variant_test_helpers.hpp

@@ -69,9 +69,9 @@ template <class Variant>
 void makeEmpty(Variant& v) {
     Variant v2(std::in_place_type<MakeEmptyT>);
     try {
-        v = v2;
+        v = std::move(v2);
         assert(false);
-    }  catch (...) {
+    } catch (...) {
         assert(v.valueless_by_exception());
     }
 }