[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
10 files changed