Implement -Wunused-label.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@114315 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Sema/SemaStmt.cpp b/lib/Sema/SemaStmt.cpp
index 47ea4f2..2448954 100644
--- a/lib/Sema/SemaStmt.cpp
+++ b/lib/Sema/SemaStmt.cpp
@@ -994,6 +994,7 @@
   if (LabelDecl == 0)
     LabelDecl = new (Context) LabelStmt(LabelLoc, LabelII, 0);
 
+  LabelDecl->setUsed();
   return Owned(new (Context) GotoStmt(LabelDecl, GotoLoc, LabelLoc));
 }