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>