Fix IORing build for older Linux kernels

[jrflat]

System currently fails to build on Ubuntu 20.04 (Linux kernel v5.4) as shown in #243, but those same tests should now pass with this PR. I verified locally that System now builds on a Swift 6.2 Ubuntu 20.04 image.

The issue is that struct io_uring_sqe has added new properties in later kernel versions, and the Swift implementation is expecting these properties to be available on the struct.

Specifically, struct io_uring_sqe added:

• cancel_flags in v5.5
• open_flags in v5.6
• file_index in v5.15

In the future, we could do something tricky to guard the availability of these features, but for now, let's just define a fallback struct for kernel versions < 5.15 so that we can build, but otherwise not support IORing initialization at runtime (such as by throwing ENOTSUP).

This PR defines a struct swift_io_uring_sqe that is just a typedef of struct io_uring_sqe on kernel versions that IORing supports. Otherwise, we use the same fallback struct that's defined when the <linux/io_uring.h> header isn't available.

We also replace the use of IORING_FEAT_NODROP with IORing.Features.nonDroppingCompletions.rawValue, which hard-codes the value for now.

[jrflat]

@swift-ci please test

[glessard]

Thanks!

[jrflat]

@swift-ci please test