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/check_unit.cpp

@@ -31,6 +31,7 @@
 #include "toolchain/check/node_id_traversal.h"
 #include "toolchain/check/type.h"
 #include "toolchain/check/type_structure.h"
+#include "toolchain/check/unused.h"
 #include "toolchain/diagnostics/diagnostic.h"
 #include "toolchain/sem_ir/function.h"
 #include "toolchain/sem_ir/ids.h"
@@ -593,7 +594,7 @@ auto CheckUnit::FinishRun() -> void {
 
   // Pop information for the file-level scope.
   context_.sem_ir().set_top_inst_block_id(context_.inst_block_stack().Pop());
-  context_.scope_stack().Pop();
+  context_.scope_stack().Pop(/*check_unused=*/true);
 
   // Finalizes the list of exports on the IR.
   context_.inst_blocks().ReplacePlaceholder(SemIR::InstBlockId::Exports,