[LLDB] Add a child property to compliment the existing parent property (#168619)

I've been working on some scripts that evaluate the parent and child
frame. It's been very annoying that the parent frame has a property but
not the child. So I've added this to the extensions, I would've
preferred to return None, but because the existing impl returns an
invalid SBFrame, so I'm conforming to that API.

```
(lldb) script
Python Interactive Interpreter. To exit, type 'quit()', 'exit()' or Ctrl-D.
>>> lldb.frame
frame #0: 0x0000555555555200 fib.out`main
>>> lldb.frame.parent
frame #1: 0x00007ffff782a610 libc.so.6`__libc_start_call_main + 128
>>> lldb.frame.parent.child
frame #0: 0x0000555555555200 fib.out`main
```
1 file changed
tree: 89ceb64a112d9f7cf986cda6ce8de4a863f1c0d0
  1. .ci/
  2. .github/
  3. bolt/
  4. clang/
  5. clang-tools-extra/
  6. cmake/
  7. compiler-rt/
  8. cross-project-tests/
  9. flang/
  10. flang-rt/
  11. libc/
  12. libclc/
  13. libcxx/
  14. libcxxabi/
  15. libsycl/
  16. libunwind/
  17. lld/
  18. lldb/
  19. llvm/
  20. llvm-libgcc/
  21. mlir/
  22. offload/
  23. openmp/
  24. orc-rt/
  25. polly/
  26. runtimes/
  27. third-party/
  28. utils/
  29. .clang-format
  30. .clang-format-ignore
  31. .clang-tidy
  32. .git-blame-ignore-revs
  33. .gitattributes
  34. .gitignore
  35. .mailmap
  36. CODE_OF_CONDUCT.md
  37. CONTRIBUTING.md
  38. LICENSE.TXT
  39. pyproject.toml
  40. README.md
  41. SECURITY.md
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