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.
https://bugs.llvm.org/show_bug.cgi?id=42311
<rdar://problem/33159405>
Differential Revision: https://reviews.llvm.org/D63518
llvm-svn: 364464
0e82895826