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>