BitStream reader: propagate errors

The bitstream reader handles errors poorly. This has two effects:

 * Bugs in file handling (especially modules) manifest as an "unexpected end of
   file" crash
 * Users of clang as a library end up aborting because the code unconditionally
   calls `report_fatal_error`

The bitstream reader should be more resilient and return Expected / Error as
soon as an error is encountered, not way late like it does now. This patch
starts doing so and adopting the error handling where I think it makes sense.
There's plenty more to do: this patch propagates errors to be minimally useful,
and follow-ups will propagate them further and improve diagnostics.

Differential Revision:

llvm-svn: 364464
32 files changed
tree: 3fc2aa5876f36d46ae328df9b7a5ee7dfa781894
  1. .arcconfig
  2. .clang-format
  3. .clang-tidy
  4. .github/
  5. .gitignore
  7. clang-tools-extra/
  8. clang/
  9. compiler-rt/
  10. debuginfo-tests/
  11. libclc/
  12. libcxx/
  13. libcxxabi/
  14. libunwind/
  15. lld/
  16. lldb/
  17. llgo/
  18. llvm/
  19. openmp/
  20. parallel-libs/
  21. polly/
  22. pstl/

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