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

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

The primary differences to go/types/check.go are:
- use of syntax instead of go/ast package
- tracing is controlled via flag not the "trace" constant

Change-Id: I1c9998afb3e0b7e29f5b169d3a4054cf22841490
Reviewed-on: https://go-review.googlesource.com/c/go/+/304109


Reviewed-by: Robert Findley <rfindley@google.com>
Trust: Robert Griesemer <gri@golang.org>