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_codeblock.cpp

@@ -4,6 +4,7 @@
 
 #include "toolchain/check/context.h"
 #include "toolchain/check/handle.h"
+#include "toolchain/check/unused.h"
 
 namespace Carbon::Check {
 
@@ -15,7 +16,7 @@ auto HandleParseNode(Context& context, Parse::CodeBlockStartId node_id)
 }
 
 auto HandleParseNode(Context& context, Parse::CodeBlockId /*node_id*/) -> bool {
-  context.scope_stack().Pop();
+  context.scope_stack().Pop(/*check_unused=*/true);
   context.node_stack()
       .PopAndDiscardSoloNodeId<Parse::NodeKind::CodeBlockStart>();
   return true;