go/types, types2: infer minimum default type for untyped arguments

go/types, types2: infer minimum default type for untyped arguments

This implements the proposal #58671.
Must be explicitly enabled and requires proposal approval.

For #58671.

Change-Id: I150e78f4f3282d6b7cf9d90feeb5f1c5a36d8c38
Reviewed-on: https://go-review.googlesource.com/c/go/+/492835
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Robert Griesemer <gri@google.com>