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/handle_choice.cpp

@@ -12,6 +12,7 @@
 #include "toolchain/check/literal.h"
 #include "toolchain/check/name_component.h"
 #include "toolchain/check/type.h"
+#include "toolchain/check/unused.h"
 #include "toolchain/diagnostics/diagnostic.h"
 #include "toolchain/lex/token_kind.h"
 #include "toolchain/sem_ir/ids.h"
@@ -307,7 +308,7 @@ auto HandleParseNode(Context& context, Parse::ChoiceDefinitionId node_id)
   // The scopes and blocks for the choice itself.
   context.inst_block_stack().Pop();
   context.decl_introducer_state_stack().Pop<Lex::TokenKind::Choice>();
-  context.scope_stack().Pop();
+  context.scope_stack().Pop(/*check_unused=*/true);
   context.decl_name_stack().PopScope();
 
   FinishGenericDefinition(context, class_info.generic_id);