By adding a constant to ClassDecl/InterfaceDecl, we're able to remove
name reference special-casing. Use TryEvalInst on the Decl to generate
the Type. For ClassDecl, then use the generated constant for
self_type_id.
Collect the contents of an `impl` into a scope, and start doing very
basic checking for `impl` declarations and definitions.
This change adds two new `Id` types to the set of type that `NodeStack`
supports -- `ImplId` and `NameScopeId`.
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Co-authored-by: Jon Ross-Perkins <jperkins@google.com>
Consume the components of the `impl` declaration, and set up scopes for
the child elements. We don't yet build a representation for the impl
itself.
Also, add an interface type value. This is necessary so that we have a
value for the expression on the right-hand side of `as` in an `impl`.