cmd/compile/internal/types2: move NewTypeParam off of Checker

This is a port of CL 347561.

A comment was corrected both in types2 and go/types, and the
compiler adjusted for the updated NewTypeParameter function.

Change-Id: I4381f0dd8e43228e1d037c5d997d421b7838f905
Reviewed-on: https://go-review.googlesource.com/c/go/+/348574


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