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/unused.cpp

@@ -0,0 +1,54 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include "toolchain/check/unused.h"
+
+#include "toolchain/base/kind_switch.h"
+#include "toolchain/check/context.h"
+#include "toolchain/check/diagnostic_helpers.h"
+
+namespace Carbon::Check {
+
+auto CheckUnusedBinding(Context& context, SemIR::NameId name_id,
+                        const LexicalLookup::Result& result) -> void {
+  // Don't warn about the special name `_`. Other special names, such as `self`,
+  // are warned if unused. `.Self` is also excluded as it is often implicit.
+  if (name_id == SemIR::NameId::Underscore ||
+      name_id == SemIR::NameId::PeriodSelf) {
+    return;
+  }
+
+  // Don't warn about names that aren't in the current file.
+  auto decl_loc = context.insts().GetCanonicalLocId(result.inst_id);
+  if (decl_loc.kind() == SemIR::LocId::Kind::ImportIRInstId) {
+    return;
+  }
+
+  auto binding = context.insts().TryGetAs<SemIR::AnyBinding>(result.inst_id);
+  if (!binding) {
+    return;
+  }
+
+  const auto& entity_name = context.entity_names().Get(binding->entity_name_id);
+  if (entity_name.is_unused) {
+    if (result.use_loc_id.has_value()) {
+      CARBON_DIAGNOSTIC(UnusedButUsed, Error,
+                        "variable `{0}` marked `unused` but used",
+                        SemIR::NameId);
+      CARBON_DIAGNOSTIC(UnusedButUsedHere, Note, "usage here");
+      context.emitter()
+          .Build(decl_loc, UnusedButUsed, name_id)
+          .Note(result.use_loc_id, UnusedButUsedHere)
+          .Emit();
+    }
+  } else {
+    if (!result.use_loc_id.has_value() && result.is_decl_reachable) {
+      CARBON_DIAGNOSTIC(UnusedBinding, Warning, "binding `{0}` unused",
+                        SemIR::NameId);
+      context.emitter().Emit(decl_loc, UnusedBinding, name_id);
+    }
+  }
+}
+
+}  // namespace Carbon::Check