Unify DeclaredName and DesignatedName as just Identifier (#2939)

This is just a simplification: I think these different forms are getting in the way more than they're helping, particularly as I was looking into namespace functionality. The handling in semantics can be identical, providing a more uniform behavior.

toolchain/parser/parser_handle_declaration_name_and_params.cpp

@@ -12,7 +12,7 @@ static auto ParserHandleDeclarationNameAndParams(ParserContext& context,
   auto state = context.PopState();
 
   if (!context.ConsumeAndAddLeafNodeIf(TokenKind::Identifier,
-                                       ParseNodeKind::DeclaredName)) {
+                                       ParseNodeKind::Identifier)) {
     CARBON_DIAGNOSTIC(ExpectedDeclarationName, Error,
                       "`{0}` introducer should be followed by a name.",
                       TokenKind);