Add test for `@Sendable` inference through an optional type.
Add test for `@Sendable` inference through an optional type.

We previously had a bug where `@Sendable` would not be propagated to a
closure when that closure was assigned to a property or passed to a
parameter of optional `@Sendable` function type. This has already been
fixed in the type checker, but add a test to ensure that we don't
regress this behavior. rdar://77789778

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