[dev.typeparams] cmd/compile: report mismatch between types because of //go:notinheap

types2 currently ignores pragmas, so it does not catch a conversion
error when converting a pointer to a type which is NOT marked notinheap
to a pointer to a convertible type, but which is marked notinheap.

So, we specifically check for this error in transformConv() and report
it during noder2.

Change-Id: I6e9c9ee29f53fa5e490c1ac8306e2191db59eeb4
Reviewed-on: https://go-review.googlesource.com/c/go/+/333369


Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
Trust: Robert Griesemer <gri@golang.org>
Trust: Dan Scales <danscales@google.com>