[Support] Implement is_local_impl with AIX mntctl

Summary:
On AIX, we can determine whether a filesystem is remote using `mntctl`.

If the information is not found, then claim that the file is remote
(since that is the more restrictive case). Testing for the associated
interface is restored with a modified version of the unit test from
rL295768.

Reviewers: jasonliu, xingxue

Reviewed By: xingxue

Subscribers: jsji, apaprocki, Hahnfeld, zturner, krytarowski, kristina, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D58801

llvm-svn: 357333