[dev.typeparams] cmd/compile: set type parameter indices when they are bound

This is a port of CL 336249 with adjustments due to slightly
different handling of type parameter declaration in types2.

The CL also contains adjustments to the compiler front-end.

With this change it is not necessary to export type parameter
indices. Filed issue #47451 so we don't forget.

Change-Id: I2834f7be313fcb4763dff2a9058f1983ee6a81b3
Reviewed-on: https://go-review.googlesource.com/c/go/+/338192


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