[lldb][gui] Update TreeItem children's m_parent on move

Before this patch, any time TreeItem is copied in Resize method, its
parent is not updated, which can cause crashes when, for example, thread
window with multiple hierarchy levels is updated. Makes TreeItem
move-only, removes TreeItem's m_delegate extra self-assignment by making
it a pointer, adds code to fix up children's parent on move constructor
and operator=
Patch prepared by NH5pml30

~~~

Huawei RRI, OS Lab

Reviewed By: clayborg

Differential Revision: https://reviews.llvm.org/D157960