exception-handling code when unassociated taskframes are present.

-) Do not remove single resumes that follow the taskframe.resume of an
 unassociated taskframe.

-) When optimizing taskframes, recognize that implicit synchronization
 occurs when control transfers to a landingpad.  This implicit
 synchronization can imply that an unassociated taskframe in the
 continuation of a spawn may be necessary, even if it does not
 explicitly contain a sync.

…fe to speculatively execute

…simplification pass

…g fails to transform a Tapir loop

…d be ignored for race detection

… ensure that the linker properly resolves those symbols in tools where only those functions, and no other hooks, are defined.

…ontain Tapir tasks. This commit addresses issue OpenCilk#13