cmd/compile/internal/syntax: remove Crawl, not needed anymore (cleanup)

cmd/compile/internal/syntax: remove Crawl, not needed anymore (cleanup)

This also brings some of the types2 testing code better in sync with
go/types.

Also: fix a minor bug in resolver_test.go (continue traversing
      SelectorExpr if the first part is not an identifier).

Change-Id: Ib6c5f6228812b49c185b52a4f02ca5b393418e01
Reviewed-on: https://go-review.googlesource.com/c/go/+/460760
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>