Issue Description:

From #26037

Currently for some tests that check for file paths, one has to add ugly patterns for the directory separators in order for the check to pass on Windows.

There has been some attempts to solve this in an unique place, like #25546 and #24940 but they are either not covering all the cases, or they generate false positives and cannot be used.

The solution might be improve FileCheck itself to support some kind of marker that expands to the right pattern.