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/semantics/semantics_handle_function.cpp

@@ -34,14 +34,12 @@ auto SemanticsHandleFunctionDefinitionStart(SemanticsContext& context,
   }
   auto param_refs_id = context.node_stack().Pop<SemanticsNodeBlockId>(
       ParseNodeKind::ParameterList);
-  auto name_node =
-      context.node_stack().PopForSoloParseNode(ParseNodeKind::DeclaredName);
+  auto [name_node, name_id] =
+      context.node_stack().PopWithParseNode<SemanticsStringId>(
+          ParseNodeKind::Identifier);
   auto fn_node = context.node_stack().PopForSoloParseNode(
       ParseNodeKind::FunctionIntroducer);
 
-  auto name_str = context.parse_tree().GetNodeText(name_node);
-  auto name_id = context.semantics_ir().AddString(name_str);
-
   // Create the entry block.
   auto outer_block = context.node_block_stack().PeekForAdd();
   context.node_block_stack().Push();