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/experimental/string.view/string.view.ops/compare.pointer.pass.cpp

@@ -72,7 +72,7 @@ int main()
     test(L"abcdefghijklmnopqrst", L"abcdefghijklmnopqrst", 0);
     }
 
-#if __cplusplus >= 201103L
+#if TEST_STD_VER >= 11
     {
     test(U"", U"", 0);
     test(U"", U"abcde", -5);