path/filepath: don't drop .. elements when cleaning invalid Windows paths

path/filepath: don't drop .. elements when cleaning invalid Windows paths

Fix a bug where Clean could improperly drop .. elements from a
path on Windows, when the path contains elements containing a ':'.

For example, Clean("a/../b:/../../c") now correctly returns "..\c"
rather than "c".

Fixes #61866

Change-Id: I97b0238953c183b2ce19ca89c14f26700008ea72
Reviewed-on: https://go-review.googlesource.com/c/go/+/517216
Run-TryBot: Damien Neil <dneil@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Quim Muntal <quimmuntal@gmail.com>