[lit] Set shlibpath_var on AIX

[hubert.reinterpretcast]

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.

[xingxue]

LGTM. Thanks!

test/Unit/lit.cfg.py

 
 def find_shlibpath_var():
     if platform.system() in ['Linux', 'FreeBSD', 'NetBSD', 'SunOS']:
         yield 'LD_LIBRARY_PATH'
     elif platform.system() == 'Darwin':
         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
     # while llvm_libs_dir is installed LLVM (and possibly older clang)
     shlibpath = os.path.pathsep.join(
         (config.shlibdir,
          config.llvm_libs_dir,
          config.environment.get(shlibpath_var, '')))
     config.environment[shlibpath_var] = shlibpath
     break
 else:
     lit_config.warning("unable to inject shared library path on '{}'"
                        .format(platform.system()))