Use numericCast() to get the correct type when creating a dispatch_block_flags_t.
Use numericCast() to get the correct type when creating a dispatch_block_flags_t.

rdar://problem/44957560

apple/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch