cmd/compile/internal/types2: merge Instantiate and InstantiateLazy

Instantiate and InstantiateLazy have the same signature; on first
principles, if Instantiate should work for importers it should be
possible to consolidate these APIs.

This CL does this. In order to make it work, a typMap needs to be
threaded through type expansion to prevent infinite recursion in the
case that the Checker is nil.

Notably, Named types now must be expanded before returning from
Underlying(). This makes Underlying generally unsafe to call while type
checking a package, so a helper function safeUnderlying is added to
provide the previous behavior. This is probably overly conservative at
most call sites, but cleanup is deferred to a later CL.

Change-Id: I03cfb75bea0750862cd6eea4e3cdc875a7daa989
Reviewed-on: https://go-review.googlesource.com/c/go/+/341855


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>