[libc] delete hdrgen (#117220)

Thanks to the effort of @RoseZhang03 and @aaryanshukla under the
guidance of
@michaelrj-google and @amykhuang, we now have newhdrgen and no longer
have a
dependency on TableGen and thus LLVM in order to start bootstrapping a
full
build.

This PR removes:
- LIBC_HDRGEN_EXE; the in tree newhdrgen is the only hdrgen that can be
used.
- LIBC_USE_NEW_HEADER_GEN; newhdrgen is the default and only option.
- LIBC_HDRGEN_ONLY; there is no need to have a distinct build step for
old
  hdrgen.
- libc-api-test and libc-api-test-tidy build targets.
- Deletes all .td files.

It does not rename newhdrgen to just hdrgen. Will follow up with a
distinct PR
for that.

Link: #117209
Link: #117254
Fixes: #117208
38 files changed
tree: a21728764630e8b139e317aa4605ee9f53cbd233
  1. .ci/
  2. .github/
  3. bolt/
  4. clang/
  5. clang-tools-extra/
  6. cmake/
  7. compiler-rt/
  8. cross-project-tests/
  9. flang/
  10. libc/
  11. libclc/
  12. libcxx/
  13. libcxxabi/
  14. libunwind/
  15. lld/
  16. lldb/
  17. llvm/
  18. llvm-libgcc/
  19. mlir/
  20. offload/
  21. openmp/
  22. polly/
  23. pstl/
  24. runtimes/
  25. third-party/
  26. utils/
  27. .clang-format
  28. .clang-tidy
  29. .git-blame-ignore-revs
  30. .gitattributes
  31. .gitignore
  32. .mailmap
  33. CODE_OF_CONDUCT.md
  34. CONTRIBUTING.md
  35. LICENSE.TXT
  36. pyproject.toml
  37. README.md
  38. SECURITY.md
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