Instead of requiring exact type match, allow assignment conversions
(those conversions allowed in the language spec without a cast) on the
returned values.

Particularly useful when the type being returned is an interface type,
but the Value actually returned is a concrete value implementing that
type (as it is tricky to return a Value which has interface type).

RELNOTE=y

Fixes #28761

Change-Id: I69eef07ca51690b2086dfa1eb549db5e4724c657
Reviewed-on: https://go-review.googlesource.com/c/go/+/174531
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>