go/types, types2: correctly include comparable in type set intersection

The comparable bit was handled incorrectly. This CL establishes
a clear invariant for a type set's terms and its comparable bit
and correctly uses the bit when computing term intersections.

Relevant changes:

- Introduce a new function intersectTermLists that does the
  correct intersection computation.

Minor:

- Moved the comparable bit after terms in _TypeSet to make it
  clearer that they belong together.

- Simplify and clarify _TypeSet.IsAll predicate.

- Remove the IsTypeSet predicate which was only used for error
  reporting in union.go, and use the existing predicates instead.

- Rename/introduce local variables in computeInterfaceTypeSet
  for consistency and to avoid confusion.

- Update some tests whose output has changed because the comparable
  bit is now only set if we have have the set of all types.
  For instance, for interface{comparable; int} the type set doesn't
  set the comparable bit because the intersection of comparable and
  int is just int; etc.

- Add many more comments to make the code clearer.

Fixes #51472.

Change-Id: I8a5661eb1693a41a17ce5f70d7e10774301f38ab
Reviewed-on: https://go-review.googlesource.com/c/go/+/390025


Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>