MFC r356005:

MFC r356005:

Merge commit f97936fab from llvm git (by Eric Fiselier):

  [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