When restoring syms, we must also restore the original Lastlineno.
Bug introduced with https://golang.org/cl/41390/.

Fixes #20415.

Change-Id: Ie81d36279d717e330951b52f42dcee4b0025b9f0
Reviewed-on: https://go-review.googlesource.com/43811Reviewed-by: Matthew Dempsky <mdempsky@google.com>