Fix default-hook synthesis on Cilksan's instrumentation pass

[neboat]

This PR fixes some issues with Cilksan's instrumentation pass.

• When synthesizing default instrumentation hooks, make sure the synthesized hook has a ret instruction that returns an appropriate value if the synthesized hook does not return void.
• If a hook has been synthesized previously, go ahead and return that hook. This way, synthesized hooks don't end up with lots of unnecessary dead basic blocks.
• Search for debug information to attach to __csan_task and __csan_detach_continue hooks, in case such debug information isn't available exactly at the hook-insertion point.

These changes fix various compiler crashes with Cilksan, including those in #291.