Implement reference counting of I/O descriptions

[alexcrichton]

This commit changes the descriptor_table.h internals, headers, etc, to reference-count the entries that are inserted. The goal of this work is to lay the foundation for #807. At this time there's no synchronization around the descriptor table for WASIp2/WASIp3 because there need not be, but with coop threads this will no longer be optional. For example it'll be possible for a thread to call close while another thread is blocked in read on that, and the result shouldn't be UB such as a trap.

At the very least code needs to execute at the start of I/O operations to flag that certain descriptors are in-use, and code needs to run at the end of I/O operations to indicate that the descriptor is no longer in-use. Today that's not possible because descriptors are borrowed raw from the table with no modifications. Reference-counting descriptors is intended to match the underlying implementation on Unix-style operating systems which is intended to enable more natural implementations of eventual semantics for concurrent I/O operations. After this commit concurrent I/O ops are still not safe (e.g. may trap, cause UB, etc), and work will be done in subsequent commits to fix this for I/O subsystems.

This commit starts out by adding a reference counter to the start of all I/O descriptions that are allocated. This is managed by descriptor_table_* functions with incs/decs and a deallocation when the count reaches 0. The table itself holds a strong reference count to all items, and then whenever something is looked up within the table it additionally gets a strong reference count returned. This is radically different from today's APIs where the descriptor_table_entry_t* points directly within the global table. The problem, then, is that all previous callers of descriptor_table_get_ref need to get updated to descriptor_table_get plus a deallocation routine.

Personally I do not have faith in myself manually placing deallocation routines. What I've opted to do here is to raise the Clang baseline for WASIp2/WASIp3 targets to Clang 22. With Clang 22 the -fdefer-ts flag is now available and the defer keyword can be used, massively simplifying the decrement of the reference count path (and resource cleanup in general). I've not touched WASIp1 here mostly because I just don't want to ruffle feathers. If someone else wants to use defer for WASIp1 I think that's fine, but I'm not going to be the one to introduce it.

The end result of this commit is that all I/O descriptions have a reference count and nothing borrows directly from the global I/O table itself. All I/O operations have a local strongly-owned copy of the description that they're working on. This now simplifies the problem of concurrent I/O to "figure it out within this object" as opposed to "is your object even valid?". I plan to do further work after this PR to audit various subsystems for modifications to I/O descriptions, add locks where appropriate, etc. Notably the descriptor table itself won't need locks in coop threading mode, just the I/O objects themselves will need internal synchronization/assertions/etc.

One note to call out is that testing this behavior is kind of hard. The consequence of a leak (forgotten dec ref) is that nothing happens. Everything generally proceeds as expected, so there's no easy way to write a test that everything's actually cleaned up. I've opted to add a debug-mode-only assertion to libc itself on program exit (hooked via crt1-command.c) that closes all descriptors and then asserts that the only live allocations are the stdio descriptors. This is intended to at least work with wasi-libc's test suite for detecting leaks (and caught one during development).

A final note is that this additionally opens up the door to implement dup on WASIp2/WASIp3 targets. I think this'll be a useful tidbit for POSIX compat (see #802) and will have a mostly trivial implementation after this PR lands.

[alexcrichton]

r? @TartanLlama