cmd/compile: simplify noding const declarations

By grouping all the logic into constDecl, we're able to get rid of the
lastconst and lasttype globals, and simplify the logic slightly. Still
clunky, but much easier to reason about.

Change-Id: I446696c31084b3bfc1fd5d3651655a81ddd159ab
Reviewed-on: https://go-review.googlesource.com/36023


Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>