go/types: better error when converting untyped values in assignments
go/types: better error when converting untyped values in assignments

The error returned by convertUntyped is 'cannot convert _ to _', which
can be misleading in contexts where an explicit conversion would be
allowed.

Arguably the error message from convertUntyped should just be 'cannot
use _ as _', as 'convert' has an explicit meaning within the spec.
Making that change caused a large number of test failures, so for now we
just fix this for assignments by interpreting the error.

For #22070

Change-Id: I4eed6f39d1a991e8df7e035ec301d28a05150eb5
Reviewed-on: https://go-review.googlesource.com/c/go/+/242083
Run-TryBot: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>

golang/go,cloudflare/go,cloudflare/go,cloudflare/go,golang/go,golang/go,nawawi/go,golang/go,CAFxX/go,cloudflare/go,CAFxX/go,nawawi/go,cloudflare/go,nawawi/go,CAFxX/go,golang/go,golang/go,nawawi/go,golang/go,golang/go,nawawi/go,CAFxX/go,CAFxX/go,cloudflare/go,nawawi/go,nawawi/go,cloudflare/go,nawawi/go,CAFxX/go,CAFxX/go,CAFxX/go,cloudflare/go