[libcxx] Use a type that is always an aggregate in variant's tests

Summary:
In PR39232, we noticed that some variant tests started failing in C++2a mode
with recent Clangs, because the rules for literal types changed in C++2a. As
a result, a temporary fix was checked in (enabling the test only in C++17).

This commit is what I believe should be the long term fix: I removed the
tests that checked constexpr default-constructibility with a weird type
from the tests for index() and valueless_by_exception(), and instead I
added tests for those using an obviously literal type in the test for the
default constructor.

Reviewers: EricWF, mclow.lists

Subscribers: christof, jkorous, dexonsmith, arphaman, libcxx-commits, rsmith

Differential Revision: https://reviews.llvm.org/D54767

llvm-svn: 347568