[dev.typeparams] go/types: use Checker-provided type parameter IDs when possible

Incrementing type parameter subscripts for each type checking pass is
distracting for an interactive program where packages are type-checked
on each keystroke.

We should perhaps hide the type parameter ID altogether, but for now at
least add a layer of indirection so that type parameters for a single
type-checked package can be stabilized.

This change should have no effect on non-generic type checking.

For #46003

Change-Id: I60d747e0a2bfb68e7d64e897eac23f609a2a4429
Reviewed-on: https://go-review.googlesource.com/c/go/+/321269


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