libclang/CIndexer.cpp: Use loadquery() on AIX for path to library

Summary:
`dladdr` is not available on AIX. Similar functionality is presented
through `loadquery`. This patch replaces a use of `dladdr` with a
version based on `loadquery`.

Reviewers: sfertile, xingxue, jasonliu

Reviewed By: xingxue

Subscribers: jsji, lhames, majnemer, asb, arphaman, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D59233

llvm-svn: 356843