go/types, types2: use "unknown field f in struct literal of type S" in error messages

go/types, types2: use "unknown field f in struct literal of type S" in error messages

This is a compromise of the error reported by the compiler (quotes
around field name removed) and the error reported by the type checkers
(added mention of struct type).

For #55326.

Change-Id: Iac4fb5c717f17c6713e90d327d39e68d3be40074
Reviewed-on: https://go-review.googlesource.com/c/go/+/434815
Reviewed-by: Robert Findley <rfindley@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>