[patch] Use the DiagnosticHandler to print diagnostics when reading bitcode.

[Rafael Espíndola]

>> +    M = std::move(*MOrErr);
>
> It's probably better to leave the `if (hasError()) return 1` path in
> here, just to future proof against changes to the diagnosticHandler.
>

The operator* of ErrorOr has an assert, so this is pretty future proof.

Cheers,
Rafael