io: free Windows operation data with _aligned_free()
io: free Windows operation data with _aligned_free()

`_dispatch_operation_perform()` uses `_aligned_malloc()` on Windows to
allocate the data buffer. This means that we cannot use
`DISPATCH_DATA_DESTRUCTOR_FREE` when we create the `dispatch_data_t`.
Provide a custom destructor which correctly frees the buffer.

Discovered while porting the dispatch_io test to Windows.

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