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

The changes between (equivalent, and reviewed) go/types/check_test.go
and check_test.go can be seen by comparing patchset 1 and 2. The actual
changes are removing the "// UNREVIEWED" marker, and minor adjustments
to get the code slightly closer to go/types/check_test.go.

The primary differences compared to go/types are:
- use of syntax rather than go/ast package
- re-implemented mechanism for error matching and elimination
  based on the syntax.ErrorMap mechanism (there's no exported
  access to the syntax scanner)
- error matching permits for column tolerances because types2
  column information doesn't match go/types column information

Change-Id: I8ae6bc93dfa2b517673b642064a1f09166755286
Reviewed-on: https://go-review.googlesource.com/c/go/+/305573


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>