[libc++] Cleanup and enable multiple warnings.
Too many warnings are being disabled too quickly. Warnings are
important to keeping libc++ correct. This patch re-enables two
warnings: -Wconstant-evaluated and -Wdeprecated-copy.
In future, all warnings disabled for the test suite should require an
attached bug. The bug should state the plan for re-enabling that
warning, or a strong case why it should remain disabled.
This should fix a number of new g++ 9 warnings.
Requested by: rlibby
MFC after: 3 days
cd0c3137f8