[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