go/types, types2: report an error when using a broken alias

The type checker doesn't have a general mechanism to "use" the type
of a type alias whose type depends on a recursive type declaration
which is not yet completely type-checked. In some cases, the type of
a type alias is needed before it is determined; the type is incorrect
(invalid) in that case but no error is reported. The type-checker is
happy with this (incorrect type), but the compiler may crash under
some circumstances.

A correct fix will likely require some form of forwarding type which
is a fairly pervasive change and may also affect the type checker API.

This CL introduces a simple side table, a map of broken type aliases,
which is consulted before the type associated with a type alias is
used. If the type alias is broken, an error is reported.

This is a stop-gap solution that prevents the compiler from crashing.
The reported error refers to the corresponding issue which suggests
a work-around that may be applicable in some cases.

Also fix a minor error related to type cycles: If we have a cycle
that doesn't start with a type, don't use a compiler error message
that explicitly mentions "type".

Fixes #50259.
Fixes #50276.
Fixes #50779.

For #50729.

Change-Id: Ie8e38f49ef724e742e8e78625e6d4f3d4014a52c
Reviewed-on: https://go-review.googlesource.com/c/go/+/379916


Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>