cmd/compile/internal/importer: lazier unified IR importing

cmd/compile/internal/importer: lazier unified IR importing

No need to eagerly read the object dictionary or setup the object
reader outside of the lazy resolve function.

Change-Id: Ic4245b0c09f3beaff97860d7f2dfb5b2b5778cc9
Reviewed-on: https://go-review.googlesource.com/c/go/+/388615
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Trust: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>