go/types: record types for union subexpressions

Prior to unions, unary and binary expressions always had a recorded
type. Preserve this by recording a type for all unary and binary
expressions encountered while parsing a union type.

Updates #50093

Change-Id: I5ba20f37854760596350d91ea325dc98e67e115a
Reviewed-on: https://go-review.googlesource.com/c/go/+/371757


Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>