[lit] Fix bug where `lit.util.which()` would return a directory

instead of executable if the argument was found inside a directory
contained in PATH.

An example where this could cause a problem is if there was a RUN line
that ran the ``test`` command and if the user had a directory in their
PATH that contained a directory called ``test/`` (that occured before
``/usr/bin/``). Lit would try to use the directory as the executable
which would fail with the rather cryptic message.

```
Could not create process due to [Errno 13] Permission denied
```

llvm-svn: 253031

utils/lit/lit/util.py

@@ -94,7 +94,7 @@ def which(command, paths = None):
     for path in paths.split(os.pathsep):
         for ext in pathext:
             p = os.path.join(path, command + ext)
-            if os.path.exists(p):
+            if os.path.exists(p) and not os.path.isdir(p):
                 return p
 
     return None