[LIT] replace `lit.util.mkdir` with `pathlib.Path.mkdir` (#163948)

`lit.util.mkdir` and `lit.util.mkdir_p` were written during the Python
2.x era.
Since modern `pathlib` functions have similar functionality, we can
simply use those instead.

If you encounter a path length issue after this change, the registry
value `LongPathsEnabled` must be set as described in
https://learn.microsoft.com/en-us/windows/win32/fileio/maximum-file-path-limitation
. Note that the Python runtime is already marked as a `longPathAware`
executable.

**Background:**

On Cygwin, a file named `file_name.exe` can be accessed without the
suffix, simply as `file_name`, as shown below:

```
$ echo > file_name.exe

$ file file_name.exe
file_name.exe: very short file (no magic)

$ file file_name
file_name: very short file (no magic)
```

In this situation, while running `mkdir file_name` works as intended,
checking for the existence of the target before calling `mkdir`
incorrectly reports that it already exists and thus skips the directory
creation.

```
$ test -e file_name && echo exists
exists

$ mkdir file_name && echo ok
ok

$ file file_name
file_name: directory
```

Therefore, the existence pre-check should be skipped on Cygwin. Instead
of add a workaround, refactored them.