go/types: flip the default value of GODEBUG=gotypesalias=1

go/types: flip the default value of GODEBUG=gotypesalias=1

This CL changes the interpretation of the unset value
of gotypesalias to equal "1". The actual deletion of
all the transitional logic will happen in a follow-up.

Note that the compiler still interprets unset as "0".
More work appears to be required within the compiler
before it is safe to flip its default.

Change-Id: I854ab1fd856c7c361a757676b0670e2f23402816
Reviewed-on: https://go-review.googlesource.com/c/go/+/577715
Reviewed-by: Robert Findley <rfindley@google.com>
Auto-Submit: Alan Donovan <adonovan@google.com>
Commit-Queue: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>