`llvm::Error`s containing errors must be explicitly handled or an assert
will be raised. With this change, `ol_impl_result_t` can accept and
consume an `llvm::Error` for errors raised by PluginInterface that have
multiple causes and other places now call `llvm::consumeError`.
Note that there is currently no facility for PluginInterface to
communicate exact error codes, but the constructor is designed in
such a way that it can be easily added later. This MR is to
convert a crash into an error code.
A new test was added, however due to the aforementioned issue with
error codes, it does not pass and instead is marked as a skip.
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