cmd/compile/internal/types2: review of resolver.go

The changes between (equivalent, and reviewed) go/types/resolver.go
and resolver.go can be seen by comparing patchset 1 and 2. The actual
changes are removing the "// UNREVIEWED" marker.

The primary differences compared to go/types are:
- use of syntax rather than go/ast package
- not using a walkDecl abstraction for const/var/type declarations

Change-Id: Id8d7b069813149ca4eadbb61d1124b22c56a91b1
Reviewed-on: https://go-review.googlesource.com/c/go/+/305572


Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>