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

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

The primary differences to go/types/expr.go are:
- use of package syntax rather than ast
- no reporting of error codes in errors
- implicit conversions of untyped nil lead to a typed nil
  (in go/types, nil remains untyped)

Change-Id: I1e235b20ebda597eb7ce597d1749f26431addde2
Reviewed-on: https://go-review.googlesource.com/c/go/+/303092


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