cmd/compile: export/import materialized aliases
This CL changes the compiler's type import/export logic
to create and preserve materialized Alias types
when GODEBUG=gotypesaliases=1.
In conjunction with CL 574717, it allows the x/tools
tests to pass with GODEBUG=gotypesaliases=1.
Updates #65294
Updates #64581
Fixes #66550
Change-Id: I70b9279f4e0ae7a1f95ad153c4e6909a878915a4
Reviewed-on: https://go-review.googlesource.com/c/go/+/574737
Auto-Submit: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>