[dev.unified] cmd/compile: write RTTI into unified IR export data

[dev.unified] cmd/compile: write RTTI into unified IR export data

This CL adds `rtype` methods for unified IR for writing/reading types
that need to have their *runtime._type value available.

For now, this just builds on the existing type writing/reading
mechanics and calling reflectdata.TypePtrAt; but longer term, reading
of derived types can be changed to use dictionary lookups instead.

Change-Id: I6f803b84546fa7df2877a8a3bcbf2623e4b03449
Reviewed-on: https://go-review.googlesource.com/c/go/+/419456
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: David Chase <drchase@google.com>