compiler, go/internal/gccgoimporter: export notinheap annotation

compiler, go/internal/gccgoimporter: export notinheap annotation

This is the gofrontend version of https://golang.org/cl/259297.
This is required now because that change is in the 1.15.3 release.

This requires changing the go/internal/gccgoimporter package, to skip
the new annotation. This change will need to be ported to the gc and
x/tools repos.

For golang/go#41761

Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/265258