[JITLink] Don't try to abandon non-existent allocations.

If JITLinkGeneric::linkPhase2 receives an Error rather than an InFlightAlloc
then we need to call JITLinkContext::notifyFailed, rather than calling
abandonAllocAndBailOut -- the latter asserts that there is an allocation to
abandon, and this was turning allocation errors into assertion failures in
debug mode.