go/types, types2: consider type parameters for cycle detection

go/types, types2: consider type parameters for cycle detection

In validType, when we see an instantiated type, proceed as with
non-generic types but provide an environment in which to look up
the values (the corresponding type arguments) of type parameters
of the instantiated type. For each type parameter for which there
is a type argument, proceed with validating that type argument.
This corresponds to applying validType to the instantiated type
without actually instantiating the type (and running into infinite
instantiations in case of invalid recursive types).

Also, when creating a type instance, use the correct source position
for the instance (the start of the qualified identifier if we have an
imported type).

Fixes #48962.

Change-Id: I196c78bf066e4a56284d53368b2eb71bd8d8a780
Reviewed-on: https://go-review.googlesource.com/c/go/+/379414
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>