Remove cv qualifiers from member pointers in the __member_pointer_traits test.  This was causing a const-qualified bind result to malfunction.  This was a recent regression due to the new use of __member_pointer_traits in restricting the __invokable and __invoke_of tests.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@181935 91177308-0d34-0410-b5e6-96231b3b80d8