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/experimental/string.view/string.view.ops/basic_string.pass.cpp

@@ -16,6 +16,8 @@
 #include <experimental/string_view>
 #include <cassert>
 
+#include "test_macros.h"
+
 template<typename CharT>
 void test ( const CharT *s ) {
     typedef std::experimental::basic_string_view<CharT> string_view_t;
@@ -49,7 +51,7 @@ int main () {
     test ( L"a" );
     test ( L"" );
 
-#if __cplusplus >= 201103L
+#if TEST_STD_VER >= 11
     test ( u"ABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDE" );
     test ( u"ABCDE" );
     test ( u"a" );