[LLDB] Check type before creating `std::atomic` synthetic children (#163176)
From
https://github.com/llvm/llvm-project/pull/163077#issuecomment-3396435083:
Currently, `std::atomic<T>` will always use the MSVC STL synthetic
children and summary. When inspecting types from other STLs, the output
would not show any children.
This PR adds a check that `std::atomic` contains `_Storage` to be
classified as coming from MSVC's STL.