[libc++] Optimize __hash_table copy constructors and assignment (#151951)

```
----------------------------------------------------------------------------------------------------------------------
Benchmark                                                                                             old          new
----------------------------------------------------------------------------------------------------------------------
std::unordered_set<int>::ctor(const&)/0                                                           15.4 ns      14.6 ns
std::unordered_set<int>::ctor(const&)/32                                                           686 ns       322 ns
std::unordered_set<int>::ctor(const&)/1024                                                       35839 ns     21490 ns
std::unordered_set<int>::ctor(const&)/8192                                                      385790 ns    280270 ns
std::unordered_set<int>::operator=(const&) (into cleared Container)/0                             15.1 ns      15.9 ns
std::unordered_set<int>::operator=(const&) (into cleared Container)/32                            1077 ns       333 ns
std::unordered_set<int>::operator=(const&) (into cleared Container)/1024                         31296 ns      9984 ns
std::unordered_set<int>::operator=(const&) (into cleared Container)/8192                        266776 ns    109418 ns
std::unordered_set<int>::operator=(const&) (into partially populated Container)/0                 15.1 ns      16.3 ns
std::unordered_set<int>::operator=(const&) (into partially populated Container)/32                 962 ns       320 ns
std::unordered_set<int>::operator=(const&) (into partially populated Container)/1024             31713 ns     10128 ns
std::unordered_set<int>::operator=(const&) (into partially populated Container)/8192            266113 ns    108525 ns
std::unordered_set<int>::operator=(const&) (into populated Container)/0                          0.990 ns      2.03 ns
std::unordered_set<int>::operator=(const&) (into populated Container)/32                           963 ns       263 ns
std::unordered_set<int>::operator=(const&) (into populated Container)/1024                       27600 ns      7793 ns
std::unordered_set<int>::operator=(const&) (into populated Container)/8192                      235295 ns     66248 ns
std::unordered_set<std::string>::ctor(const&)/0                                                   16.0 ns      15.0 ns
std::unordered_set<std::string>::ctor(const&)/32                                                  2950 ns      1277 ns
std::unordered_set<std::string>::ctor(const&)/1024                                              246935 ns     73762 ns
std::unordered_set<std::string>::ctor(const&)/8192                                             3310895 ns   2468608 ns
std::unordered_set<std::string>::operator=(const&) (into cleared Container)/0                     16.1 ns      15.8 ns
std::unordered_set<std::string>::operator=(const&) (into cleared Container)/32                    5856 ns      1039 ns
std::unordered_set<std::string>::operator=(const&) (into cleared Container)/1024                170436 ns     74836 ns
std::unordered_set<std::string>::operator=(const&) (into cleared Container)/8192               1574235 ns   1096891 ns
std::unordered_set<std::string>::operator=(const&) (into partially populated Container)/0         16.0 ns      16.3 ns
std::unordered_set<std::string>::operator=(const&) (into partially populated Container)/32        5571 ns      1064 ns
std::unordered_set<std::string>::operator=(const&) (into partially populated Container)/1024    199220 ns     75462 ns
std::unordered_set<std::string>::operator=(const&) (into partially populated Container)/8192   1552465 ns   1116094 ns
std::unordered_set<std::string>::operator=(const&) (into populated Container)/0                   1.70 ns      2.14 ns
std::unordered_set<std::string>::operator=(const&) (into populated Container)/32                  2562 ns       645 ns
std::unordered_set<std::string>::operator=(const&) (into populated Container)/1024              228608 ns     39100 ns
std::unordered_set<std::string>::operator=(const&) (into populated Container)/8192             2013723 ns    390401 ns
```

Fixes #77657
16 files changed
tree: 792309a9f79daeb044238721711aab1020454a9d
  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. polly/
  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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