cmd/compile/internal/types2: record types for union subexpressions

This is a port of CL 371757 from go/types to types2, with
minor adjustments for different error handling and AST.

It also names the added API test cases more consistently.
The same renaming was applied to the respective go/types
file.

Updates #50093

Change-Id: Iaa132106a197a207f831525432e62e9d452b17c9
Reviewed-on: https://go-review.googlesource.com/c/go/+/372475


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