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.access/data.pass.cpp

@@ -15,6 +15,8 @@
 #include <experimental/string_view>
 #include <cassert>
 
+#include "test_macros.h"
+
 template <typename CharT>
 void test ( const CharT *s, size_t len ) {
     std::experimental::basic_string_view<CharT> sv ( s, len );
@@ -29,7 +31,7 @@ int main () {
     test ( L"ABCDE", 5 );
     test ( L"a", 1 );
 
-#if __cplusplus >= 201103L
+#if TEST_STD_VER >= 11
     test ( u"ABCDE", 5 );
     test ( u"a", 1 );