Releasing wasi-sdk-31

[alexcrichton]

Similar to #587 I'd like to open an issue ahead of time about wasi-sdk-31. The current changelog includes:

• Source-level support for C++ exceptions (no current plans yet for continuous binary artifacts, see Support for C++ exceptions #565 for more info)
• Update to LLVM 22.1.0
• Source-level support for wasip3 as a target (no current plans for continuous binary artifacts, see WASIp3, threading ABI changes, LLVM releases, and schedules #589 for more info)
• Build libclang.so as part of the sysroot
• Build LLDB as part of the sysroot
• Clang's default target is now wasm32-wasip1 instead of wasm32-wasi

The changelog for wasi-libc includes:

• A number of fixes for networking in WASIp2 - don't mark just-connected socket ready-to-read in poll(2) wasi-libc#732, support non-blocking wasip2 sockets in read and write wasi-libc#734, only signal read (not write) readiness for listening sockets wasi-libc#737, report read readiness accurately for freshly bound UDP sockets wasi-libc#740, implement F_{GET,SET}FL for fcntl for sockets wasi-libc#742
• Work towards a more complete wasip3 target implementation, but it's not fully done yet.

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The main headline feature of this release, in my mind, is LLVM 22.1.0. That fixes issues like #595 and is an update over the prior LLVM 21.x.0 version. That's also why I wanted to get this released after the wasi-sdk-30 release so soon, too.

As with prior releases, wanted to open this for comments/thoughts if anyone else has objections or other things they'd like to see included.