Change test suite to support c++17 dialect flag instead of c++1z.

This patch changes the test suite to attempt and prefer -std=c++17 over
-std=c++1z. It also fixes the REQUIRES and UNSUPPORTED lit markers
to refer to c++17 over c++1z.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@317610 91177308-0d34-0410-b5e6-96231b3b80d8
5 files changed