[lldb][DataFormatters] Adjust retrieval of unordered_map element type (#140256)

A user ran into an issue where the libc++ `std::unordered_map` formatter
fails because it can't deduce the `element_type`. That happens because
the `node_type` is a forwad declaration. And, in fact, dsymutil stripped
the definition for `std::__1::__hash_node<...>` for a particular
instantiation. While I'm still unclear whether this is a dsymutil bug,
this patch works around said issue by getting the element type from the
`__table_` member.

Drive-by:
- Set the `m_element_type` in `Update`, which is where the other members
are initialized

I don't have a reduced example of this unfortunately. But the crux of
the issue is that `std::__1::__hash_node<...>` only has a forward
declaration in the dsym. Then trying to call `GetTypeTemplateArgument`
on that `CompilerType` fails. And even if the definition was present in
the dsym it seems like we're stopped in a context where the CU only had
a forward declaration DIE for that type and the `node_type` never ends
up being completed with the definition that lives in another CU.

rdar://150813798
1 file changed
tree: 38e8742a3e700b9057eabb964747c73d16a9410f
  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. libunwind/
  16. lld/
  17. lldb/
  18. llvm/
  19. llvm-libgcc/
  20. mlir/
  21. offload/
  22. openmp/
  23. polly/
  24. pstl/
  25. runtimes/
  26. third-party/
  27. utils/
  28. .clang-format
  29. .clang-format-ignore
  30. .clang-tidy
  31. .git-blame-ignore-revs
  32. .gitattributes
  33. .gitignore
  34. .mailmap
  35. CODE_OF_CONDUCT.md
  36. CONTRIBUTING.md
  37. LICENSE.TXT
  38. pyproject.toml
  39. README.md
  40. SECURITY.md
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