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.nonmembers/string.io/stream_insert.pass.cpp

@@ -52,7 +52,7 @@ int main()
         assert(out.good());
         assert(L"   " + s == out.str());
     }
-#if __cplusplus >= 201103L
+#if TEST_STD_VER >= 11
     {
         typedef std::basic_string<char, std::char_traits<char>, min_allocator<char>> S;
         std::basic_ostringstream<S::value_type, S::traits_type, S::allocator_type> out;