make zig compiler processes live across rebuilds

[andrewrk]

$ zig build --watch -fincremental
Build Summary: 5/5 steps succeeded
install success
├─ install abc success
│  └─ zig build-lib abc Debug native success 183ms
└─ install abc success
   └─ zig build-exe abc Debug native success 195ms
watching 67 directories, 2 processes

Closes #20600

---

This gets the build system fully ready for incremental compilation. The next step is to finish incremental compilation features in the frontend, as you can see from using the results of this PR:

andy@bark ~/t/abc [SIGINT]> ~/dev/zig/build-release/stage4/bin/zig build --watch -fincremental
Build Summary: 5/5 steps succeeded
install success
├─ install abc success
│  └─ zig build-lib abc Debug native success 1s
└─ install abc success
   └─ zig build-exe abc Debug native success 1s
install
└─ install abc
   └─ zig build-exe abc Debug native failure
error: thread 91237 panic: access of union field 'pl_node' while field 'declaration' is active
/home/andy/dev/zig/lib/std/zig/Zir.zig:4027:53: 0x653b360 in getAssociatedSrcHash (zig)
            const pl_node = data[@intFromEnum(inst)].pl_node;
                                                    ^
/home/andy/dev/zig/src/Zcu/PerThread.zig:379:45: 0x653a1f7 in updateZirRefs (zig)
            if (old_zir.getAssociatedSrcHash(old_inst)) |old_hash| hash_changed: {
                                            ^
/home/andy/dev/zig/src/Zcu/PerThread.zig:324:29: 0x654033e in astGenFile (zig)
        try pt.updateZirRefs(file, file_index, prev_zir.*);
                            ^
/home/andy/dev/zig/src/Compilation.zig:4309:18: 0x60a5c52 in workerAstGenFile (zig)
    pt.astGenFile(file, file_index, path_digest, root_decl) catch |err| switch (err) {
                 ^
/home/andy/dev/zig/lib/std/Thread/Pool.zig:178:50: 0x60a64cc in runFn (zig)
            @call(.auto, func, .{id.?} ++ closure.arguments);
                                                 ^
/home/andy/dev/zig/lib/std/Thread/Pool.zig:291:32: 0x6064b8d in worker (zig)
            run_node.data.runFn(&run_node.data, id);
                               ^
/home/andy/dev/zig/lib/std/Thread.zig:409:13: 0x5d837ba in callFn__anon_73354 (zig)
            @call(.auto, f, args);
            ^
/home/andy/dev/zig/lib/std/Thread.zig:678:30: 0x5b016e2 in entryFn (zig)
                return callFn(f, args_ptr.*);
                             ^
???:?:?: 0x7f318dada271 in ??? (libc.so.6)
Unwind information for `libc.so.6:0x7f318dada271` was not available, trace may be incomplete


error: the following command terminated unexpectedly:
/home/andy/dev/zig/build-release/stage4/bin/zig build-exe -fno-llvm -fno-lld -ODebug -Mroot=/home/andy/tmp/abc/src/main.zig --cache-dir /home/andy/tmp/abc/.zig-cache --global-cache-dir /home/andy/.cache/zig --name abc --zig-lib-dir /home/andy/dev/zig/lib/ -fincremental --listen=- 
Build Summary: 2/5 steps succeeded; 1 failed
install transitive failure
└─ install abc transitive failure
   └─ zig build-exe abc Debug native failure
watching 67 directories, 1 processes

Here, the build system has kept the compiler running, and asked for an update. The frontend crashed due to a bug/missing feature.

Users should not use -fincremental yet, in order to avoid triggering these bugs. If you don't pass -fincremental then it doesn't keep the compiler alive.