Zcu: use shortest reference trace

[mlugg]

The logic for computing reference traces was unintentionally finding the longest possible trace (approximately). I think I already tried to fix this before, but misunderstood how my own code works. Here, we fix it properly: by slightly reworking the logic to use one ArrayHashMap for both the result and the traversal queue, we trivially get a proper breadth-first traversal so that we can find the shortest possible reference trace for every referenced unit.

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For instance, let's add a @compileError into the compiler internals (Sema.resolveValue for no particular reason) and look at the reference trace before and after this change.

Before:

referenced by:
    resolvePeerTypes: src/Sema.zig:33175:38
    zirResolveInferredAlloc: src/Sema.zig:4394:60
    analyzeBodyInner: src/Sema.zig:1161:78
    analyzeInlineBody: src/Sema.zig:1062:30
    unionFields: src/Sema.zig:35657:39
    resolveUnionFieldTypes: src/Sema.zig:35059:21
    resolveUnionInner: src/Type.zig:3877:47
    resolveFields: src/Type.zig:3704:55
    bitSizeInner__anon_473482: src/Type.zig:1716:37
    abiSizeInner__anon_473150: src/Type.zig:1340:96
    abiAlignmentInner__anon_473148: src/Type.zig:1003:112
    abiAlignmentSema: src/Type.zig:947:37
    print: src/print_value.zig:83:69
    format: src/print_value.zig:36:17
    format [inlined]: lib/std/fmt.zig:274:25
    printValue__anon_482873: lib/std/Io/Writer.zig:1050:39
    print__anon_333281: lib/std/Io/Writer.zig:701:25
    bufPrint__anon_264139: lib/std/fmt.zig:599:12
    idleProgNode: src/link/Elf2.zig:1635:39
    idle: src/link/Elf2.zig:1556:51
    idle: src/link.zig:868:76
    doIdleTask: src/link.zig:1610:43
    flushTaskQueue: src/link/Queue.zig:239:46
    runFn: lib/std/Thread/Pool.zig:180:13
    Thread.Pool.spawnWgId__anon_227885.Closure: lib/std/Thread/Pool.zig:175:34
    spawnWgId__anon_227885: lib/std/Thread/Pool.zig:172:21
    start: src/link/Queue.zig:112:35
    performAllTheWork: src/Compilation.zig:4617:31
    update: src/Compilation.zig:3094:31
    serve: src/main.zig:4115:32
    buildOutputType: src/main.zig:3547:22
    mainArgs: src/main.zig:252:31
    main: src/main.zig:204:20
    callMain [inlined]: lib/std/start.zig:697:37
    callMainWithArgs [inlined]: lib/std/start.zig:657:20
    posixCallMainAndExit: lib/std/start.zig:612:36
    _start: lib/std/start.zig:499:40
    comptime: lib/std/start.zig:96:71
    start: lib/std/std.zig:108:27
    comptime: lib/std/std.zig:195:9

After:

referenced by:
    resolveConstValue: src/Sema.zig:2267:33
    resolveFinalDeclValue: src/Sema.zig:2312:43
    analyzeNavVal: src/Zcu/PerThread.zig:1176:54
    ensureNavValUpToDate: src/Zcu/PerThread.zig:1008:80
    processOneJob: src/Compilation.zig:5162:58
    performAllTheWork: src/Compilation.zig:5017:30
    update: src/Compilation.zig:3094:31
    jitCmd: src/main.zig:5530:28
    mainArgs: src/main.zig:300:22
    main: src/main.zig:204:20
    callMain [inlined]: lib/std/start.zig:697:37
    callMainWithArgs [inlined]: lib/std/start.zig:657:20
    posixCallMainAndExit: lib/std/start.zig:612:36
    _start: lib/std/start.zig:499:40
    comptime: lib/std/start.zig:96:71
    start: lib/std/std.zig:108:27
    comptime: lib/std/std.zig:195:9