Use TEST_STD_VER instead of __cplusplus [NFC]

While __cplusplus was only used a few dozen times, TEST_STD_VAR is used
more than 2000 times. So we replace the former by the latter for
consistency in the tests. There should be no functional change.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@344194 91177308-0d34-0410-b5e6-96231b3b80d8
24 files changed