[release-branch.go1.12] path/filepath: don't discard .. in EvalSymlinks

EvalSymlinks was mishandling cases like "/x/../../y" or "../../../x"
where there is an extra ".." that goes past the start of the path.

Updates #30520
Fixes #30586

Change-Id: I07525575f83009032fa1a99aa270c8d42007d276
Reviewed-on: https://go-review.googlesource.com/c/go/+/164762


Reviewed-by: Bryan C. Mills <bcmills@google.com>
(cherry picked from commit 294edb27)
Reviewed-on: https://go-review.googlesource.com/c/go/+/165197


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>