cmSystemTools: Restore GetRealPathResolvingWindowsSubst

We need it to restore behavior on Windows.  Revert commit 16af27fd42
(cmSystemTools: Drop GetRealPathResolvingWindowsSubst, 2024-11-04,
v4.0.0-rc1~521^2), but with a cleaned up implementation.

Issue: #26750
Issue: #18033
3 files changed