swift234

Sema: Fix for setter->isMutating() check before setter was type checked

With an upcoming patch we would call setMutating() on materializeForSet
before computing the setter's isMutating() in the case where a setter
was explicitly declared 'nonmutating'.

Fix that by replacing the setter->isMutating() call with a direct
computation of the expected result.

It seems that the materializeForSet of protocol protocol requirements
has to be mutating, even if the protocol is a class protocol or the
property is nonmutating -- I need to investigate why and fix SILGen
to not make this assumption, but in the meantime, opt-out of the
new logic with protocol requirements to avoid more breakage.