Implement unused pattern bindings, continued (#6518)

Implementation of unused pattern bindings #2022, continued.

Whereas previous PR #6460 took care of parsing, and PR #6479 prepared
the stage by using _ in some test cases, this PR has the the actual
implementation, using a simple dataflow analysis.

---------

Co-authored-by: Burak Emir <bqe@google.com>
Co-authored-by: jonmeow <jperkins@google.com>

toolchain/check/interface.cpp

@@ -139,7 +139,8 @@ auto AddSelfSymbolicBindingToScope(Context& context,
                                    bool is_template) -> SemIR::InstId {
   auto entity_name_id = context.entity_names().AddSymbolicBindingName(
       SemIR::NameId::SelfType, scope_id,
-      context.scope_stack().AddCompileTimeBinding(), is_template);
+      context.scope_stack().AddCompileTimeBinding(), is_template,
+      /*is_unused=*/false);
   // Because there is no equivalent non-symbolic value, we use `None` as
   // the `value_id` on the `SymbolicBinding`.
   auto self_param_inst_id =