[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