[libcxx] Reorganize tests since the application of P0602R4

Summary:
P0602R4 makes the special member functions of optional and variant
conditionally trivial based on the types in the optional/variant.
We already implemented that, but the tests were organized as if this
were a non-standard extension. This patch reorganizes the tests in a
way that makes more sense since this is not an extension anymore.

Reviewers: EricWF, mpark, mclow.lists

Subscribers: christof, jkorous, dexonsmith, libcxx-commits

Differential Revision: https://reviews.llvm.org/D54772

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@350884 91177308-0d34-0410-b5e6-96231b3b80d8
13 files changed