cmd/compile: ensure we replace package placeholder in type names
We want package names exposed by reflect to be things like
main.F[main.foo], not main.F["".foo].
Fixes #49547
Change-Id: I182411a75d56ce1f64fde847e5b9ee74ce44e00b
Reviewed-on: https://go-review.googlesource.com/c/go/+/363656
Trust: Keith Randall <khr@golang.org>
Trust: Dan Scales <danscales@google.com>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Go Bot <gobot@golang.org> Reviewed-by: Dan Scales <danscales@google.com>