go/types: if base type for an alias is known, use it when needed

Because methods are type-checked before the receiver base type
is "complete" (i.e., they are checked as part of the receiver
base type), situations occur where aliases of those base types
are used (in those methods) but the alias types are not known
yet (even though their base types are known).

This fix is a temporary work-around that looks syntactically
for the base types of alias types and uses those base types
when we refer to an "incomplete" alias type. The work-around
is completely localized and guarded with a flag so it can be
disabled at short notice.

The correct fix (slated for 1.12) is to decouple type-checking
of methods from their receiver base types. See issue #26854.

Fixes #26390.

Change-Id: I66cc9d834b220c254ac00e671a137cf8a3da59c1
Reviewed-on: https://go-review.googlesource.com/128435


Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>