[ORC] Add errors for missing and extraneous symbol definitions.

This patch adds new errors and error checking to the ObjectLinkingLayer to
catch cases where a compiled or loaded object either:
(1) Contains definitions not covered by its responsibility set, or
(2) Is missing definitions that are covered by its responsibility set.

Proir to this patch providing the correct set of definitions was treated as
an API contract requirement, however this requires that the client be confident
in the correctness of the whole compiler / object-cache pipeline and results
in difficult-to-debug assertions upon failure. Treating this as a recoverable
error results in clearer diagnostics.

The performance overhead of this check is one comparison of densemap keys
(symbol string pointers) per linking object, which is minimal. If this overhead
ever becomes a problem we can add the check under a flag that can be turned off
if the client fully trusts the rest of the pipeline.