cmd/compile: tighten exportWriter.qualifiedIdent signature

It always takes an *ir.Name and would panic given anything else
anyway, so tighten the signature to match.

Change-Id: I44fec5c5cc6d7f0b9c59eb91c9f9633137485360
Reviewed-on: https://go-review.googlesource.com/c/go/+/314009


Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>