Validate parse nodes correspond to expected tokens (#3295)

Can specify which tokens are allowed generally, and any additional
tokens that only occur when the parse node has an error.

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Co-authored-by: Richard Smith <richard@metafoo.co.uk>

toolchain/parse/handle_declaration_name_and_params.cpp

@@ -33,7 +33,8 @@ static auto HandleDeclarationNameAndParams(Context& context, State after_name)
     context.emitter().Emit(*context.position(), ExpectedDeclarationName,
                            context.tokens().GetKind(state.token));
     context.ReturnErrorOnState();
-    context.AddLeafNode(NodeKind::InvalidParse, *context.position());
+    context.AddLeafNode(NodeKind::InvalidParse, *context.position(),
+                        /*has_error=*/true);
   }
 }