[Sema] Do not emit -Wunused-variable for variables declared with cleanup attribute

[nathanchance]

Summary

A variable declared with __attribute__((cleanup)) cannot be unused, as
its address is passed to the clean up function. Do not emit
-Wunused-variable for variables declared with the cleanup attribute,
which matches GCC's behavior: https://godbolt.org/z/dz5YfTsan

[nickdesaulniers]

Thanks for the patch!

clang/test/Sema/warn-unused-variables.c

35

void return type

[nathanchance]

Change f4()'s return type to void (Nick)

[erichkeane]

I note I forgot to require a release note, so please add one of those (and make sure it gets incorporated in any cherry-pick that happens).

clang/lib/Sema/SemaDecl.cpp

for (auto *BD : DD->bindings())
       if (BD->isReferenced())
         return false;
   } else if (!D->getDeclName()) {
     return false;
   } else if (D->isReferenced() || D->isUsed()) {
     return false;
   }
 
-  if (D->hasAttr<UnusedAttr>() || D->hasAttr<ObjCPreciseLifetimeAttr>())
+  if (D->hasAttr<UnusedAttr>() || D->hasAttr<ObjCPreciseLifetimeAttr>() ||
+      D->hasAttr<CleanupAttr>())
     return false;
 
   if (isa<LabelDecl>(D))
     return true;
 
   // Except for labels, we only care about unused decls that are local to
   // functions.
   bool WithinFunction = D->getDeclContext()->isFunctionOrMethod();

clang/test/Sema/warn-unused-variables.c

 }
 
 int f3(void) {
   int X1 = 4; 
   (void)(Y1 + X1); // expected-error {{use of undeclared identifier 'Y1'}}
   (void)(^() { int X = 4; }); // expected-warning{{unused}}
   (void)(^() { int X = 4; return Y + X; }); // expected-error {{use of undeclared identifier 'Y'}}
 }
+
+void c1(int *);
+void f4(void) {
+  int __attribute__((cleanup(c1))) X1 = 4;
+}