[libcxxabi] copy back std::string_view patches from LLVM

I made a series of changes to LLVM's demangle in:
- D148348
- D148353
- D148363
- D148375
and so did Fangrui in 3ece37b3fa2c and Ashay in D149061.

I didn't notice the banner about there being two copies of this in tree
and was modifying the downstream versions. Copy these changes back to
the upstream version. Oops!

Reviewed By: MaskRay, #libc_abi, ldionne, phosek

Differential Revision: https://reviews.llvm.org/D148566
4 files changed
tree: 8376eea648bb73bb5efae107a061fcb8622c00b3
  1. .github/
  2. bolt/
  3. clang/
  4. clang-tools-extra/
  5. cmake/
  6. compiler-rt/
  7. cross-project-tests/
  8. flang/
  9. libc/
  10. libclc/
  11. libcxx/
  12. libcxxabi/
  13. libunwind/
  14. lld/
  15. lldb/
  16. llvm/
  17. llvm-libgcc/
  18. mlir/
  19. openmp/
  20. polly/
  21. pstl/
  22. runtimes/
  23. third-party/
  24. utils/
  25. .arcconfig
  26. .arclint
  27. .clang-format
  28. .clang-tidy
  29. .git-blame-ignore-revs
  30. .gitignore
  31. .mailmap
  32. CONTRIBUTING.md
  33. LICENSE.TXT
  34. README.md
  35. SECURITY.md
README.md

The LLVM Compiler Infrastructure

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