We can't follow symlinks for fear of directory cycles and other problems,
but we can at least notice potentially-relevant symlinks that are being
ignored and report them.

Fixes #17662.

Change-Id: I1fce00bd5b80ea8df45dac8b61bfa08076ec5f4b
Reviewed-on: https://go-review.googlesource.com/46425
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>