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

libcxx/test/std/strings/basic.string/string.access/index.pass.cpp

@@ -36,7 +36,7 @@ int main()
     const S s2 = S();
     assert(s2[0] == '\0');
     }
-#if __cplusplus >= 201103L
+#if TEST_STD_VER >= 11
     {
     typedef std::basic_string<char, std::char_traits<char>, min_allocator<char>> S;
     S s("0123456789");