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
diff --git a/libcxx/test/std/containers/associative/multiset/empty.pass.cpp b/libcxx/test/std/containers/associative/multiset/empty.pass.cpp
index 32aef90..acca4e0 100644
--- a/libcxx/test/std/containers/associative/multiset/empty.pass.cpp
+++ b/libcxx/test/std/containers/associative/multiset/empty.pass.cpp
@@ -29,7 +29,7 @@
     m.clear();
     assert(m.empty());
     }
-#if __cplusplus >= 201103L
+#if TEST_STD_VER >= 11
     {
     typedef std::multiset<int, std::less<int>, min_allocator<int>> M;
     M m;