Decompilation tests sometimes cannot be compiled on Windows due to unicode escapes in comments

[smarter]

As reported on Gitter by @michelou, the decompilation tests fail on Windows:

-- Error: C:\dotty\out\posTestFromTasty\pos_decompiled\clsrefine\decompiled.scala:28:63 -
28 |/** Decompiled from out\posTestFromTasty\pos\clsrefine\scalac\util\test.class */
   |                                                               ^
   |                                                               error in unicode escape

It's very weird, but Scala will try to interpret a unicode escape \uXXXX when it sees it in a comment. And since Windows paths are separated using \, this means we run into trouble as soon as a directory name starts with u. We can avoid this issue by always using / when printing filenames in comments, even on Windows.

[nicolasstucki]

Always using / seems like a good compromise.

[smarter]

Note that there's a WIP PR to change this behavior in scalac: scala/scala#6661