[CodeLayout] Fixing initialization of empty ranges (#68917)

Fixing libc++'s consistency checks, by eliminating ranges of singular
iterators.

Author: spupyrev

llvm/lib/Transforms/Utils/CodeLayout.cpp

@@ -476,13 +476,16 @@ void ChainT::mergeEdges(ChainT *Other) {
 }
 
 using NodeIter = std::vector<NodeT *>::const_iterator;
+static std::vector<NodeT *> EmptyList;
 
 /// A wrapper around three concatenated vectors (chains) of nodes; it is used
 /// to avoid extra instantiation of the vectors.
 struct MergedNodesT {
-  MergedNodesT(NodeIter Begin1, NodeIter End1, NodeIter Begin2 = NodeIter(),
-               NodeIter End2 = NodeIter(), NodeIter Begin3 = NodeIter(),
-               NodeIter End3 = NodeIter())
+  MergedNodesT(NodeIter Begin1, NodeIter End1,
+               NodeIter Begin2 = EmptyList.begin(),
+               NodeIter End2 = EmptyList.end(),
+               NodeIter Begin3 = EmptyList.begin(),
+               NodeIter End3 = EmptyList.end())
       : Begin1(Begin1), End1(End1), Begin2(Begin2), End2(End2), Begin3(Begin3),
         End3(End3) {}