[dev.typeparams] cmd/compile/internal/types2: consistently report nil type as "untyped nil"

This fixes an inconsistency where the type for nil in code such as

	var x unsafe.Pointer = nil

and in conversions of the form

	T(nil)

(where T is a pointer, function, slice, map, channel, interface, or
unsafe.Pointer) was reported as (converted to) the respective type.
For all other operations that accept a nil value, we don't do this
conversion for nil.

(We never change the type of the untyped nil value, in contrast to
other untyped values where we give the values context-specific types.)

It may still be useful to change this behavior and - consistently -
report a converted nil type like we do for any other type, but for
now this CL simply fixes the existing inconsistency.

Added tests and fixed existing test harness.

Updates #13061.

Change-Id: Ia82832845c096e3cbc4a239ba3d6c8b9a9d274c9
Reviewed-on: https://go-review.googlesource.com/c/go/+/284052


Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>