go/types, types2: eliminate need to sort arguments for type inference

go/types, types2: eliminate need to sort arguments for type inference

When unifying types, we always consider underlying types if inference
would fail otherwise. If a type parameter has a (non-defined) type
inferred and later matches against a defined type, make sure to keep
that defined type instead.

For #43056.

Change-Id: I24e4cd2939df7c8069e505be10914017c1c1c288
Reviewed-on: https://go-review.googlesource.com/c/go/+/464348
Reviewed-by: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>