[patch] Update catch(...) handlers to deal with __forced_unwind

[Jonathan Wakely]

Failing to rethrow a __forced_unwind exception is very bad.

This patch ensures we rethrow them in async tasks, and makes the
shared state ready with a broken_promise so that waiting threads
don't block forever. That seems reasonable to me, does anyone have any
better ideas?

Tested x86_64-linux, will wait for feedback before committing.

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