[libc++] Correctly handle Objective-C++ ARC qualifiers in std::is_pointer

Summary:
Otherwise, std::is_pointer<id __strong> works, but std::is_pointer<id __weak>
(and others) don't work as expected.

rdar://problem/49126333

Reviewers: ahatanak, EricWF

Subscribers: christof, jkorous, dexonsmith, libcxx-commits

Differential Revision: https://reviews.llvm.org/D60087

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@357517 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed