[func-sig-opts] When splicing BB from one function to another, change

the debug scope of each instruction to be the debug scope of the new
function.

This will be tested by the functionality in function signature opts.

Swift SVN r22553

lib/SIL/SILBasicBlock.cpp

@@ -116,6 +116,10 @@ transferNodesFromList(llvm::ilist_traits<SILBasicBlock> &SrcTraits,
   if (Parent == SrcTraits.Parent) return;
 
   // If splicing blocks not in the same function, update the parent pointers.
-  for (; First != Last; ++First)
+  for (; First != Last; ++First) {
     First->Parent = Parent;
+    for (auto &II : *First) {
+      II.setDebugScope(Parent->getDebugScope());
+    }
+  }
 }