[Clang] Remove gnuwin32 documentation references (#177557)
Remove the documentation references to GnuWin32. The project is no
longer maintained, and as LLVM is now using Git, `llvm-lit` is now using
the GNU core utilities packaged with it rather than requiring a separate
installation.
This appears to have been on
[discourse](https://discourse.llvm.org/t/gnuwin32-alternatives-for-tests-of-msvc-build/42846/3)
but not implemented yet.
