[JITLink] Ensure that in-flight alloc is abandoned on error in post-alloc phase.

If an error occurs during the post-allocation phase (JITLinkerBase::linkPhase2)
then we need to call JITLinkMemoryManager::InFlightAlloc::abandon so that the
allocation has a chance to clean up. This was already handled for later phases,
but we were skipping the abandon step when we bailed out of phase 2.