[clang] Mark labels referenced when used in named break or continue (llvm#166033)

Fixes llvm#166013 Marks labels that appear in a c2y named break or continue statement as referenced to fix false-positive unused diagnostics. --------- Co-authored-by: camc <pushy-crop-cartel@duck.com>

Author: camc

clang/docs/ReleaseNotes.rst

@@ -395,6 +395,8 @@ Improvements to Clang's diagnostics
  that were previously incorrectly accepted in case of other irrelevant
  conditions are now consistently diagnosed, identical to C++ mode.
 
+- Fix false-positive unused label diagnostic when a label is used in a named break
+ or continue (#GH166013)
 - Clang now emits a diagnostic in case `vector_size` or `ext_vector_type`
  attributes are used with a negative size (#GH165463).
 

clang/lib/Sema/SemaStmt.cpp

@@ -3281,6 +3281,9 @@ static Scope *FindLabeledBreakContinueScope(Sema &S, Scope *CurScope,
  SourceLocation LabelLoc,
  bool IsContinue) {
  assert(Target && "not a named break/continue?");
+
+ Target->markUsed(S.Context);
+
  Scope *Found = nullptr;
  for (Scope *Scope = CurScope; Scope; Scope = Scope->getParent()) {
  if (Scope->isFunctionScope())

clang/test/Sema/labeled-break-continue.c

@@ -1,6 +1,6 @@
-// RUN: %clang_cc1 -std=c2y -verify -fsyntax-only -fblocks %s
-// RUN: %clang_cc1 -std=c23 -verify -fsyntax-only -fblocks -fnamed-loops %s
-// RUN: %clang_cc1 -x c++ -verify -fsyntax-only -fblocks -fnamed-loops %s
+// RUN: %clang_cc1 -std=c2y -verify -Wunused -fsyntax-only -fblocks %s
+// RUN: %clang_cc1 -std=c23 -verify -Wunused -fsyntax-only -fblocks -fnamed-loops %s
+// RUN: %clang_cc1 -x c++ -verify -Wunused -fsyntax-only -fblocks -fnamed-loops %s
 
 void f1() {
  l1: while (true) {
@@ -159,3 +159,15 @@ void f7() {
  continue d; // expected-error {{'continue' label does not name an enclosing loop}}
  }
 }
+
+void f8() {
+ l1: // no-warning
+ while (true) {
+ break l1;
+ }
+
+ l2: // no-warning
+ while (true) {
+ continue l2;
+ }
+}