[windows] Use both directory separators in test patterns.

[drodriguez]

Admit defeat and realize that one simply cannot win against Windows. Use
the pattern with both separators in the tests and remove the hack in
PathSanitizingFileCheck because it created false positives, some of them
could not have been easily fixed.

Sadly lit substitutions or other tricks in lit.cfg will not work,
because CHECK lines are not processed by lit, but by FileCheck.

[drodriguez]

@swift-ci please smoke test

[gottesmm]

@drodriguez question. Maybe add support for this upstream to FileCheck?

[drodriguez]

@gottesmm: it doesn’t really matter. There's some moments that you want to check that \ is matched (for example https://github.com/apple/swift/blob/379d88cb122447bdd507b835e2f7b651ec09c6a0/test/Parse/raw_string.swift#L111). In those cases, one should not do the replacement, but it completely looks like a Windows path. It is sad, and I cringe every time I see the {{\\|/}}, but I think the only way of removing them is ensuring every piece emits Unix paths, even on Windows.

@swift-ci please smoke test Linux platform

[gottesmm]

This really sucks. We need to find a better way to do this.