[Demangler] Fix assertion failure.
It's illegal to call `node->addChild()` with a `NULL` child argument;
it's possible to construct unexpected `Node` trees by passing invalid
manglings, and in this case that was causing `popTypeAndGetChild()` to
fail (because the top node was not a `Type` node), which then meant
that the call to `addChild` had a `NULL` child argument.
The simplest fix is to use `createWithChildren()` to do the node
construction, because that function checks its arguments for `NULL`s.
rdar://125350219
