swift234

add a new ContinueAfterFailures default argument to ConstraintSystem::simplify,
and use it in the diagnostics path (only!) to revisit active constraints that
are left in the system after a failure is found.  This improves a number of 
otherwise sad diagnostics in the testsuite and resolves rdar://22083115.

The one QoI regression (in throwing_functions.swift) is now tracked by 22158167.



Swift SVN r31027