path/filepath: make Join("c:", "/a") return "c:/a" again

path/filepath: make Join("c:", "/a") return "c:/a" again

Historically, on Windows filepath.Join("c:", elt) does not insert
a path separator between "c:" and elt, but preserves leading slashes
in elt. Restore this behavior, which was inadvertently changed by
CL 444280.

Fixes #56988

Change-Id: Id728bf311f4093264f8c067d8b801ea9ebef5b5f
Reviewed-on: https://go-review.googlesource.com/c/go/+/453497
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Run-TryBot: Damien Neil <dneil@google.com>