runtime: make the old remangler allocation free.

Extract common code from the old and new remangler into a common base class.
This lets the old remangler benefit from the changes I did recently in the new remangler.

Author: eeckstein

include/swift/Demangling/Demangle.h

@@ -506,9 +506,20 @@ llvm::StringRef mangleNode(NodePointer root, SymbolicResolver resolver,
 /// Remangle in the old mangling scheme.
 ///
 /// This is only used for objc-runtime names.
-/// If \p BorrowFrom is specified, the initial bump pointer memory is
-/// borrowed from the free memory of BorrowFrom.
-std::string mangleNodeOld(NodePointer root, NodeFactory *BorrowFrom = nullptr);
+std::string mangleNodeOld(NodePointer root);
+
+/// Remangle in the old mangling scheme.
+///
+/// This is only used for objc-runtime names.
+/// The returned string is owned by \p Factory. This means \p Factory must stay
+/// alive as long as the returned string is used.
+llvm::StringRef mangleNodeOld(NodePointer node, NodeFactory &Factory);
+
+/// Remangle in the old mangling scheme and embed the name in "_Tt<name>_".
+///
+/// The returned string is null terminated and owned by \p Factory. This means
+/// \p Factory must stay alive as long as the returned string is used.
+const char *mangleNodeAsObjcCString(NodePointer node, NodeFactory &Factory);
 
 /// Transform the node structure to a string.
 ///