[dev.typeparams] go/types, types2: revert fancy struct printing (fixes x/tools tests)

An embedded struct field is embedded by mentioning its type.
The fact that the field name may be different and derived
from the type doesn't matter for the struct type.

Do print the embedded type rather than the derived field
name, as we have always done in the past. Remove the fancy
new code which was just plain wrong.

The struct output printing is only relevant for debugging
and test cases. Reverting to the original code (pre-generics)
fixes a couple of x/tools tests.

Unfortunately, the original code is (also) not correct for
embedded type aliases. Adjusted a gccgoimporter test
accordingly and filed issue #44410.

This is a follow-up on https://golang.org/cl/293961 which
addressed the issue only partially and left the incorrect
code in place.

Change-Id: Icb7a89c12ef7929c221fb1a5792f144f7fcd5855
Reviewed-on: https://go-review.googlesource.com/c/go/+/293962


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>