Don't set HasPotentialFeatureAvailabilityViolations in ActOnLabelStmt

Set it in ActOnObjCAvailabilityCheckExpr instead. Without this change,
DiagnoseUnguardedFeatureAvailability scans the whole function when there
is a label in it even when there are no Decls annotated with feature
availability that are referenced in the function and there are no
ObjCAvailabilityCheckExprs.

rdar://146139827

Author: ahatanaka

clang/lib/Sema/SemaExprObjC.cpp

@@ -5154,6 +5154,8 @@ ExprResult SemaObjC::ActOnObjCAvailabilityCheckExpr(
   ASTContext &Context = getASTContext();
 
   if (AvailSpecs.front().isDomainName()) {
+    if (FunctionScopeInfo *Info = SemaRef.getCurFunctionAvailabilityContext())
+      Info->HasPotentialFeatureAvailabilityViolations = true;
     auto Spec = AvailSpecs.front();
     return ObjCAvailabilityCheckExpr::CreateAvailabilityFeatureCheck(
         AtLoc, RParen, Context.BoolTy, Spec.getDomainName(), Context);

clang/lib/Sema/SemaStmt.cpp

@@ -602,9 +602,6 @@ Sema::ActOnLabelStmt(SourceLocation IdentLoc, LabelDecl *TheDecl,
     }
   }
 
-  if (FunctionScopeInfo *Info = getCurFunctionAvailabilityContext())
-    Info->HasPotentialFeatureAvailabilityViolations = true;
-
   return LS;
 }