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

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

The primary differences to go/types/assignments.go are:
- use of syntax instead of go/ast package
- no reporting of error codes (for now)
- different handling of nil values (we can't use Typ[UntypedNil]
  to represent an untyped nil because types2 gives such nil values
  context-dependent types)

Change-Id: I5d8a58f43ca8ed2daa060c46842a6ebc11b3cb35
Reviewed-on: https://go-review.googlesource.com/c/go/+/304051


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