Replace __cplusplus comparisons and dialect __has_feature checks with TEST_STD_VER.

This is a huge cleanup that helps make the libc++ test suite more portable.
Patch from STL@microsoft.com. Thanks STL!

llvm-svn: 272716
GitOrigin-RevId: f2f2a6395fad5bd49a573fdf2b20072735d496f7
827 files changed