[dev.typeparams] go/types: conversions to type parameters are not constant

This is a port of CL 290471 to go/types. However, this change preserves
the existing check for constant types in recordTypeAndValue, which uses
is(..., isConstType) rather than the isConstType predicate. In types2,
this code path is not hit with type parameters because convertUntyped
walks the type list in order before calling updateExprType with the type
parameter, at which point the expression type would have already been
recorded as the first element of the type list -- probably something
that should be corrected.

Longer term, I believe we actually could allow const type parameters if
the optype is a sum of constant types.

Change-Id: Iaa91ffa740b5f08a5696bd96918a866bffd7aef6
Reviewed-on: https://go-review.googlesource.com/c/go/+/291323


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