cmd/compile/internal/types2: use a global atomic counter for type parameter ids

This avoids a dependency on a *Checker when we create type parameters
outside the type checker proper, e.g. in an importer. There may be
better solutions but this does the trick for now.

Change-Id: Icf22c934970cb04c88c2729555ae6a79ef5a2245
Reviewed-on: https://go-review.googlesource.com/c/go/+/309830


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