Scaffolding for checking impls. (#3672)

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`.

toolchain/check/node_stack.h

@@ -492,6 +492,7 @@ class NodeStack {
           return IdKind::InstId;
         case Parse::NodeKind::IfCondition:
         case Parse::NodeKind::IfExprIf:
+        case Parse::NodeKind::ImplForall:
         case Parse::NodeKind::ImplicitParamList:
         case Parse::NodeKind::TuplePattern:
         case Parse::NodeKind::WhileCondition:
@@ -513,6 +514,7 @@ class NodeStack {
         case Parse::NodeKind::FunctionIntroducer:
         case Parse::NodeKind::IfStatementElse:
         case Parse::NodeKind::ImplicitParamListStart:
+        case Parse::NodeKind::ImplIntroducer:
         case Parse::NodeKind::InterfaceIntroducer:
         case Parse::NodeKind::LetIntroducer:
         case Parse::NodeKind::QualifiedName: