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!


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@272716 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/strings/basic.string/string.access/back.pass.cpp

@@ -39,7 +39,7 @@ int main()
     test(S("1"));
     test(S("1234567890123456789012345678901234567890"));
     }
-#if __cplusplus >= 201103L
+#if TEST_STD_VER >= 11
     {
     typedef std::basic_string<char, std::char_traits<char>, min_allocator<char>> S;
     test(S("1"));