go/types, types2: implement adjCoreType using TypeParam.is

TypeParam.is also provides ~ (tilde) information which is needed
to fix #51229. Delete all code related to singleType as it's not
used anymore.

Also, remove TypeParam.hasTerms as it was not used.

For #51229.

Change-Id: Ie49b19d157230beecb17a444d1f17cf24aa4f6ba
Reviewed-on: https://go-review.googlesource.com/c/go/+/387774


Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>