cmd/compile: allow aliases to go:notinheap types

The alias doesn't need to be marked go:notinheap. It gets its
notinheap-ness from the target type.

Without this change, the type alias test in the notinheap.go file
generates these two errors:

notinheap.go:62: misplaced compiler directive
notinheap.go:63: type nih must be go:notinheap

The first is a result of go:notinheap pragmas not applying
to type alias declarations.
The second is the result of then trying to match the notinheap-ness
of the alias and the target type.

Add a few more go:notinheap tests while we are here.

Update #40954

Change-Id: I067ec47698df6e9e593e080d67796fd05a1d480f
Reviewed-on: https://go-review.googlesource.com/c/go/+/250939


Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
Trust: Keith Randall <khr@golang.org>