SR-3477: replace unsafeBitCast by withoutActuallyEscaping
SR-3477: replace unsafeBitCast by withoutActuallyEscaping

Now that withoutActuallyEscaping is implemented, use it instead
of unsafeBitCast to allow passing a closure capturing a non-escaping
block down to the C implementation of dispatch_data_apply (which does
not actually escape its block argument).

Signed-off-by: Daniel A. Steffen <bc823475b60dbcbd7be1bbdfe095b5f1939d5bd2@apple.com>

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