cmd/compile/internal/types2: implement types2.Instantiate

Instantiation support for imports. This is experimental
but it also doesn't affect Go 1.17 as this code is not
executed unless we enable generics (in the parser).

Change-Id: If2da09ac3a557ec6a180707a53f75f3ce354f3e9
Reviewed-on: https://go-review.googlesource.com/c/go/+/314773


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