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_struct.cpp

@@ -8,6 +8,7 @@
 #include "toolchain/check/handle.h"
 #include "toolchain/check/inst.h"
 #include "toolchain/check/type.h"
+#include "toolchain/check/unused.h"
 #include "toolchain/diagnostics/format_providers.h"
 
 namespace Carbon::Check {
@@ -137,7 +138,7 @@ auto HandleParseNode(Context& context, Parse::StructLiteralId node_id) -> bool {
   auto elements_id = context.param_and_arg_refs_stack().EndAndPop(
       Parse::NodeKind::StructLiteralStart);
 
-  context.scope_stack().Pop();
+  context.scope_stack().Pop(/*check_unused=*/true);
   context.node_stack()
       .PopAndDiscardSoloNodeId<Parse::NodeKind::StructLiteralStart>();
 
@@ -163,7 +164,7 @@ auto HandleParseNode(Context& context, Parse::StructTypeLiteralId node_id)
   llvm::SmallVector<Parse::NodeId> field_name_nodes =
       PopFieldNameNodes(context, fields.size());
 
-  context.scope_stack().Pop();
+  context.scope_stack().Pop(/*check_unused=*/true);
   context.node_stack()
       .PopAndDiscardSoloNodeId<Parse::NodeKind::StructTypeLiteralStart>();