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/sequences/vector/contiguous.pass.cpp

@@ -40,7 +40,7 @@ int main()
     test_contiguous(C(A(3)));
     test_contiguous(C(7, 9.0, A(5)));
     }
-#if __cplusplus >= 201103L
+#if TEST_STD_VER >= 11
     {
     typedef double T;
     typedef min_allocator<T> A;