go/types: instantiate methods when instantiating Named types

In the API proposal we decided that instantiation must also instantiate
methods. This CL does that, and eliminates the special handling for lazy
instantiation in lookupMethod.

It is possible that we expand an instance before all method signatures
have been type-checked, so for simplicity we introduce a new flag on
Func, 'isIncompleteMethod', which controls whether we must fully
substitute methods before using them. We could avoid this flag by using
some convention for the structure of an incomplete method (such as the
receiver has no position), but in practice using a flag was cleaner and
didn't increase the size of the Func struct.

Updates #47916

Change-Id: I352baa6664cd07f61b06924744382897805f9d29
Reviewed-on: https://go-review.googlesource.com/c/go/+/349412


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>