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_where.cpp

@@ -9,6 +9,7 @@
 #include "toolchain/check/handle.h"
 #include "toolchain/check/inst.h"
 #include "toolchain/check/type.h"
+#include "toolchain/check/unused.h"
 #include "toolchain/sem_ir/facet_type_info.h"
 #include "toolchain/sem_ir/ids.h"
 #include "toolchain/sem_ir/inst.h"
@@ -188,7 +189,7 @@ auto HandleParseNode(Context& context, Parse::WhereExprId node_id) -> bool {
   context.rewrites_stack().pop_back();
   // Remove `PeriodSelf` from name lookup, undoing the `Push` done for the
   // `WhereOperand`.
-  context.scope_stack().Pop();
+  context.scope_stack().Pop(/*check_unused=*/true);
   SemIR::InstId period_self_id =
       context.node_stack().Pop<Parse::NodeKind::WhereOperand>();
   SemIR::InstBlockId requirements_id = context.args_type_info_stack().Pop();