Use a map to detect duplicate symbols. Allows eliminating an otherwise
unneeded field from Sym and gets rid of a global variable.

Change-Id: Ic004bca7e9130a1261a1cddbc17244529a2a1df4
Reviewed-on: https://go-review.googlesource.com/20552Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>