[lit] Set shlibpath_var on AIX

Summary:
When building the `check-all` target on AIX, lit produces
```
warning: unable to inject shared library path on 'AIX'
```

This patch addresses this. `LIBPATH` is the environment variable of
interest on AIX. Newer versions of AIX may consider `LD_LIBRARY_PATH`,
but only when `LIBPATH` is unset.

Reviewers: xingxue, jasonliu, sfertile, serge-sans-paille

Reviewed By: xingxue

Subscribers: jsji, cfe-commits, llvm-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D59741


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@357334 91177308-0d34-0410-b5e6-96231b3b80d8

test/Unit/lit.cfg.py

@@ -42,6 +42,8 @@ def find_shlibpath_var():
         yield 'DYLD_LIBRARY_PATH'
     elif platform.system() == 'Windows':
         yield 'PATH'
+    elif platform.system() == 'AIX':
+        yield 'LIBPATH'
 
 for shlibpath_var in find_shlibpath_var():
     # in stand-alone builds, shlibdir is clang's build tree