go/types: use documented version of gotypesalias GODEBUG
This way the code would panic, in case it does not exist.
Change-Id: I95de7460c0386afdc5d3f6a847e9fcbd22446010
GitHub-Last-Rev:
9ae0502a091feed45169f5c1a7e2761f8ffa2841
GitHub-Pull-Request: golang/go#70845
Reviewed-on: https://go-review.googlesource.com/c/go/+/636097
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Alan Donovan <adonovan@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>