go/types: fix bug in premature Unalias of alias cycle
Unalias memoizes the result of removing Alias constructors.
When Unalias is called too soon on a type in a cycle,
the initial value of the alias, Invalid, gets latched by
the memoization, causing it to appear Invalid forever.
This change disables memoization of Invalid, and adds
a regression test.
Fixes #66704
Updates #65294
Change-Id: I479fe14c88c802504a69f177869f091656489cd4
Reviewed-on: https://go-review.googlesource.com/c/go/+/576975
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Auto-Submit: Alan Donovan <adonovan@google.com>