[clang][Modules] Fixing Incorrect Diagnostics Issued during By-name Dependency Scanning (#178542)

The by-name lookup API uses the same diagnostics engine and consumer for
multiple lookups. When multiple lookups fail, the diagnostics could be
incorrect for all but the first failing lookup. All the subsequent
failing lookups inherit the diagnostics from the first failing lookup.

This PR resets the diagnostics consumer's buffer and the
CompilerInstance's diagnostics engine for each by-name lookup, so each
lookup can produce the correct diagnostics.

Part of work for rdar://136303612.
4 files changed
tree: 37f92bde37c5bb14709980c0049e04d4b1fbb7a3
  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. orc-rt/
  25. polly/
  26. runtimes/
  27. third-party/
  28. utils/
  29. .clang-format
  30. .clang-format-ignore
  31. .clang-tidy
  32. .git-blame-ignore-revs
  33. .gitattributes
  34. .gitignore
  35. .mailmap
  36. CODE_OF_CONDUCT.md
  37. CONTRIBUTING.md
  38. LICENSE.TXT
  39. pyproject.toml
  40. README.md
  41. SECURITY.md
README.md

The LLVM Compiler Infrastructure

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