More steps towards simpler symbol handling:

- Pushdcl's incoming pos argument, saved in a newly pushed *Sym, was always
  immediately overwritten by the Lastlineno value of the saved *Sym.

- Markdcl's incoming pos argument, saved in the stack mark *Sym, was not
  restored when the stack mark was popped.

- Popdcl always maintained the most recent Lastlineno for a *Sym given
  by package and name, making it unnecessary to save Lastlineno in the
  first place. Removed Lastlineno from the set of fields that need saving,
  and simplified Popdcl.

Change-Id: Ie93da1fbd780dcafc2703044e781c0c6298df569
Reviewed-on: https://go-review.googlesource.com/41390
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>