go/types: add struct field with invalid type if field has errors

This ensures that all struct fields are present and thus the struct
has the original number of fields even if some fields have type
errors. (This only applies as long as the field names themselves
don't conflict.)

Fixes #25627.

Change-Id: I2414b1f432ce139b3cd2776ff0d46d8dcf38b650
Reviewed-on: https://go-review.googlesource.com/115115


Reviewed-by: Alan Donovan <adonovan@google.com>