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.