Demangler: support stack allocation for the Demangler's allocator

The demangler can be initialized with a preallocated memory on the stack. Only in case of an overflow, the bump pointer allocator mallocs new memory.
Also, support that a new instance of a demangler can "borrow" the free memory from an existing demangler. This is useful because in the runtime the demangler is invoked recursively. With this feature, all the nested demanglers can share a single stack allocated space.

lib/Demangling/Demangler.cpp

@@ -408,6 +408,7 @@ void NodeFactory::freeSlabs(Slab *slab) {
 }
   
 void NodeFactory::clear() {
+  assert(!isBorrowed);
   if (CurrentSlab) {
 #ifdef NODE_FACTORY_DEBUGGING
     std::cerr << indent() << "## clear: allocated memory = " << allocatedMemory  << "\n";