Implement syntactic merge checks for parameters. (#4149)

Note this isn't implementing checking through imports. The parse node
there is harder to access through the context, so would require
examining the entity in order to get the import declaration, to get at
the ImportIR. We also don't have a parse tree attached in that case, and
would need to add one to SemIR::File. But I believe we do want to add
that, so it's explicitly a TODO.

Note GetTokenText re-lexes literal values, so there's a bit of potential
overhead there. Not sure if we want a more efficient manner for
comparing in cases like this.

toolchain/check/name_component.cpp

@@ -9,15 +9,40 @@
 namespace Carbon::Check {
 
 auto PopNameComponent(Context& context) -> NameComponent {
+  Parse::NodeId first_param_node_id = Parse::InvalidNodeId();
+  Parse::NodeId last_param_node_id = Parse::InvalidNodeId();
+
+  // Explicit params.
   auto [params_loc_id, params_id] =
       context.node_stack().PopWithNodeIdIf<Parse::NodeKind::TuplePattern>();
+  if (params_id) {
+    first_param_node_id =
+        context.node_stack()
+            .PopForSoloNodeId<Parse::NodeKind::TuplePatternStart>();
+    last_param_node_id = params_loc_id;
+  }
+
+  // Implicit params.
   auto [implicit_params_loc_id, implicit_params_id] =
       context.node_stack()
           .PopWithNodeIdIf<Parse::NodeKind::ImplicitParamList>();
+  if (implicit_params_id) {
+    // Implicit params always come before explicit params.
+    first_param_node_id =
+        context.node_stack()
+            .PopForSoloNodeId<Parse::NodeKind::ImplicitParamListStart>();
+    // Only use the end of implicit params if there weren't explicit params.
+    if (last_param_node_id.is_valid()) {
+      last_param_node_id = params_loc_id;
+    }
+  }
+
   auto [name_loc_id, name_id] = context.node_stack().PopNameWithNodeId();
   return {
       .name_loc_id = name_loc_id,
       .name_id = name_id,
+      .first_param_node_id = first_param_node_id,
+      .last_param_node_id = last_param_node_id,
       .implicit_params_loc_id = implicit_params_loc_id,
       .implicit_params_id =
           implicit_params_id.value_or(SemIR::InstBlockId::Invalid),