[lldb][test] Merge MSVC STL std::(u8)string tests into generic directory (#147525)

Now that most STL formatter tests have been moved to `generic`. Do the
same for the MSVC tests (which are currently just for `std::string`).
The `std::string` test was mostly the same (MSVC just had 1 additional
check, which I moved over).

We also only tested `u8string` with MSVC. So i moved those into
`generic` as-is. I kept it separate from the existing std::string tests
since it requires c++20.

The tests are currently failing for libc++ and libstdc++ because MSVC
had a test case which checked that:
```
  std::string overwritten_zero("abc");
  const_cast<char *>(overwritten_zero.data())[3] = 'd';
```
prints as `"abc"`. But libc++ and libstdc++ print it as `"abcd"` (which
seems like the more correct thing to do?)
10 files changed
tree: 169c85013a535b0f2c82cd8b42c0952e20ef2536
  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. runtimes/
  25. third-party/
  26. utils/
  27. .clang-format
  28. .clang-format-ignore
  29. .clang-tidy
  30. .git-blame-ignore-revs
  31. .gitattributes
  32. .gitignore
  33. .mailmap
  34. CODE_OF_CONDUCT.md
  35. CONTRIBUTING.md
  36. LICENSE.TXT
  37. pyproject.toml
  38. README.md
  39. SECURITY.md
README.md

The LLVM Compiler Infrastructure

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