[TypeChecker] SE-0326: Allow `ExprAvailabilityWalker` to walk into multi-statement closures
[TypeChecker] SE-0326: Allow `ExprAvailabilityWalker` to walk into multi-statement closures

When `experimental-multi-statement-closures` flag is enabled,
multi-statement closures are type-checked together with enclosing
context, which means that availability checking is allowed to walk
into them.

apple/swift,benlangmuir/swift,roambotics/swift,roambotics/swift,glessard/swift,gregomni/swift,gregomni/swift,JGiola/swift,gregomni/swift,glessard/swift,gregomni/swift,rudkx/swift,atrick/swift,rudkx/swift,ahoppen/swift,roambotics/swift,ahoppen/swift,benlangmuir/swift,roambotics/swift,roambotics/swift,ahoppen/swift,benlangmuir/swift,apple/swift,benlangmuir/swift,rudkx/swift,atrick/swift,atrick/swift,JGiola/swift,JGiola/swift,roambotics/swift,ahoppen/swift,rudkx/swift,gregomni/swift,apple/swift,gregomni/swift,benlangmuir/swift,glessard/swift,ahoppen/swift,ahoppen/swift,JGiola/swift,JGiola/swift,glessard/swift,benlangmuir/swift,glessard/swift,rudkx/swift,apple/swift,atrick/swift,glessard/swift,apple/swift,rudkx/swift,atrick/swift,JGiola/swift,atrick/swift,apple/swift