go/types, types2: make function type inference argument-order independent

If we have more than 2 arguments, we may have arguments with named and
unnamed types. If that is the case, permutate params and args such that
the arguments with named types are first in the list. This doesn't affect
type inference if all types are taken as is. But when we have inexact
unification enabled (as is the case for function type inference), when
a named type is unified with an unnamed type, unification proceeds with
the underlying type of the named type because otherwise unification would
fail right away. This leads to an asymmetry in type inference: in cases
where arguments of named and unnamed types are passed to parameters with
identical type, different types (named vs underlying) may be inferred
depending on the order of the arguments.
By ensuring that named types are seen first, order dependence is avoided
and unification succeeds where it can.

This CL implements the respectice code but keeps it disabled for now,
pending decision whether we want to address this issue in the first
place.

For #43056.

Change-Id: Ibe3b08ec2afe90a24a8c30cd1875d504bcc2ef39
Reviewed-on: https://go-review.googlesource.com/c/go/+/377894
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

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