Add framework for the extern keyword. (#3755)

This doesn't add full support. I'm separating it out to make the effects
of the modifier changes clearer for review. I'm restructuring a little
with the expectation that we'll have some more categories of modifier
keywords in the future (similar to `extern`, these may not be in a "set"
such as access), and thus easily scaling up to a few more would be
useful.

toolchain/check/handle_impl.cpp

@@ -209,7 +209,8 @@ static auto BuildImplDecl(Context& context, Parse::AnyImplDeclId parse_node)
   // For an `extend impl` declaration, mark the impl as extending this `impl`.
   if (!!(context.decl_state_stack().innermost().modifier_set &
          KeywordModifierSet::Extend)) {
-    auto extend_node = context.decl_state_stack().innermost().saw_decl_modifier;
+    auto extend_node = context.decl_state_stack().innermost().modifier_node_id(
+        ModifierOrder::Decl);
     ExtendImpl(context, extend_node, parse_node, self_type_node, self_type_id,
                params_node, constraint_type_id);
   }