go/types: eliminate Named.instPos

We no longer need to use the nilness of Named.instPos to signal whether
instance expansion has occurred, so remove it from the Named struct by
instead closing over the instantiation position in the resolver.

This means we cannot print instance markers for unexpanded instances:
instances may escape the type checking pass without being fully
expanded, and we can not check whether they have been expanded in a
concurrency-safe way without introducing a more heavy-weight
syncronization mechanism.

With this change, instantiation should be concurrency safe, modulo bugs
of course as we have little test coverage of concurrency (see #47729).

Fixes #47910

Change-Id: Ifeef6df296f00105579554b333a44d08aae113c9
Reviewed-on: https://go-review.googlesource.com/c/go/+/349411


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