go/types, types2: replace typecheck with mustTypecheck almost everywhere (cleanup)

go/types, types2: replace typecheck with mustTypecheck almost everywhere (cleanup)

Replace even in places where before we have a specific error message
or different control-flow (except in TestTypeString or TestObjectString)
because failing to type-check in virtually all cases represents an error
in the test itself.

Change-Id: I9f1e6d25bddd92c168353409b281b5a3f29a747c
Reviewed-on: https://go-review.googlesource.com/c/go/+/443915
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>