This CL adds support for preserving materialized aliases
while types transit export data (indexed or unified).

(This CL depends on CL 574737 to the compiler and
go/types.)

Updates golang/go#65294
Updates golang/go#64581

Change-Id: I1a0a08357e4f6a480ba6250fbea327922e455873
Reviewed-on: https://go-review.googlesource.com/c/tools/+/574717
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Findley <rfindley@google.com>

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>

This test reenables the staticcheck test, which now passes
with gotypesalias={,0,1}. (Staticcheck may yet need work, but
that isn't evident to our tests.)

Updates golang/go#64581

Change-Id: I1c8acca2c6b7d16e551a5c8a68be564254713d0d
Reviewed-on: https://go-review.googlesource.com/c/tools/+/579756
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>

The predicate that determines whether the type checker
creates types.Alias nodes is complex: it depends on an
environment variable (GODEBUG) that is somewhat tricky
to parse correctly, since it is a rightmost-wins list.

Critically, however, its default value is a function of
the go directive in the application's go.mod file, which
is inaccessible to logic in x/tools (per-file build tags
can detect the toolchain version, but not the go.mod version).
Equally critically, the current effective value of the
gotypesalias variable changes when os.Setenv(GODEBUG) is
called, which happens in tests, and there is no way to
detect those events; therefore any attempt to cache the
value is wrong.

This change exposes a simplified version of the Enabled
function from aliases, which always computes the ground
truth by invoking the type checker, regardless of cost.
It also adds an 'enabled' parameter to NewAlias so that
callers can amortize this cost.

Also, various minor cleanups related to aliases.

Updates golang/go#64581

Change-Id: If926edefb8e1c1f63c17e4fad0a808e27bac6d5b
Reviewed-on: https://go-review.googlesource.com/c/tools/+/580455
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Findley <rfindley@google.com>