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/multimap/multimap.ops/find.pass.cpp

@@ -83,7 +83,7 @@ int main()
         assert(r == m.end());
     }
     }
-#if __cplusplus >= 201103L
+#if TEST_STD_VER >= 11
     {
     typedef std::multimap<int, double, std::less<int>, min_allocator<std::pair<const int, double>>> M;
     {