[lit] Avoid os.path.realpath in lit.py due to MAX_PATH limitations on Windows

lit.py uses os.path.realpath on file paths. Somewhere between Python 3.7
and 3.9, os.path.realpath was updated to resolve substitute drives on
Windows (subst S: C:\Long\Path\To\My\Code). This is a problem because it
prevents using substitute drives to work around MAX_PATH path length
limitations on Windows.

We run into this while building & testing, the Swift compiler on
Windows, which uses a substitute drive in CI to shorten the workspace
directory. cmake builds without resolving the substitute drive and can
apply its logic to avoid output files exceeding MAX_PATH. However, when
running tests, lit.py's use of os.path.realpath will resolve the
substitute drive (with newer Python versions), resulting in some paths
being longer than MAX_PATH, which cause all kinds of failures (for
example rd in tests fails, or link.exe fails, etc).

How tested: Ran check-all, and lit tests, saw no failures
```
> ninja -C build check-all
Testing Time: 262.63s
  Skipped          :    24
  Unsupported      :  2074
  Passed           : 51812
  Expectedly Failed:   167

> python utils\lit\lit.py --path ..\build\bin utils\lit\tests
Testing Time: 12.17s
  Unsupported:  6
  Passed     : 47
```

Patch by Tristan Labelle!

Differential Revision: https://reviews.llvm.org/D152709
Reviewed By: rnk, compnerd