cmd/compile/internal/types2: use an opaque environment for Instantiate

This is a port of CL 343930 from go/types, adjusted to work for
the compiler: here Environment carries a *Checker, if available.

Change-Id: I44544fad7da870fa0c02832baa6abd2909d50304
Reviewed-on: https://go-review.googlesource.com/c/go/+/344612


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