Pavel Yaskevich 43670cb897 [TypeChecker] Classify nil literal and .none as non-contriburing to throws ...

Original fix for SR-9102 stripped throws bit from the function types
nested inside optionals before attempting bindings, that doesn't
work with e.g. default parameter values because conversions from
throwing and non-throwing functions are only allowed in subtype
relationship but function types nested inside optionals are going
to be equated.

So this patch takes an alternative approach and attempts to pattern
match `nil` literal and `.none` use and classify argument as
non-contributing to throws.

Resolves: rdar://problem/47550715

2019-01-29 10:27:37 -08:00

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