Introduce a new test macro TEST_HAS_C11_FEATURES which is set when the...

Introduce a new test macro TEST_HAS_C11_FEATURES which is set when the underlying C library has C11 features. In C++17, we use those features. <__config> defines a similar macro, _LIBCPP_HAS_C11_FEATURES, but we don't want to use that in the library-independent parts of the tests, so define the new one. Also add a libc++-specific test to make sure the two stay in sync.

llvm-svn: 338411