[libc++] Fix is_pointer support for Objective-C++

This test regressed with 5ade17e0, but we never noticed it because
.pass.mm tests were skipped due to a bug in our Lit config. This commit
fixes is_pointer (by essentially reverting tha part of 5ade17e0) and
also adds .pass.mm tests to the list of supported test suffixes.

We can explore how to support __is_pointer with Objective-C++ qualifiers
as a follow-up -- the main goal of this commit is to fix the regression
quickly and make sure all tests of the suite are run.