[LTO] Avoid assert fail on failed pass plugin load (#96691)

Without this patch, passing -load-pass-plugin=nonexistent.so to
llvm-lto2 produces a backtrace because LTOBackend.cpp does not handle
the error correctly:

```
Failed to load passes from 'nonexistant.so'. Request ignored.
Expected<T> must be checked before access or destruction.
Unchecked Expected<T> contained error:
Could not load library 'nonexistant.so': nonexistant.so: cannot open shared object file: No such file or directoryPLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace.
```

Any tool using `lto::Config::PassPlugins` should suffer similarly.

Based on the message "Request ignored" and the continue statement, the
intention was apparently to continue on failure to load a plugin.
However, no one appears to rely on that behavior now given that it
crashes instead, and terminating is consistent with opt.
2 files changed
tree: d3835593f85b5f70c9081631cce99acf0097fc60
  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
README.md

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