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/containers/associative/set/set.cons/default.pass.cpp

@@ -25,7 +25,7 @@ int main()
     assert(m.empty());
     assert(m.begin() == m.end());
     }
-#if __cplusplus >= 201103L
+#if TEST_STD_VER >= 11
     {
     std::set<int, std::less<int>, min_allocator<int>> m;
     assert(m.empty());