Fix Issue 18760 - theAllocator dangling reference bug

[edi33416]

This fixes the following bug

    import std.experimental.allocator.mallocator : Mallocator;

    auto a = sharedAllocatorObject(Mallocator.instance);
    auto buf = theAllocator.allocate(42);
    processAllocator = a;
    theAllocator.deallocate(buf);

[dlang-bot]

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Testing this PR locally

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[JackStouffer]

This should have a bug report.

[edi33416]

This should have a bug report.

I'll file one and then link this to it

[n8sh]

The auto-tester failure appears unrelated.

[edi33416]

@CyberShadow any idea why DAutoTest is failing?

[CyberShadow]

It is a random failure likely due a concurrency issue, see here:
dlang/dlang.org#2345

[edi33416]

Got it. Apparently it's kicking quite a few PRs lately.

[jercaianu]

@edi33416 can you rebase please?

[andralex]

ouch

[andralex]

@CyberShadow can you please take a look at the failure?

[CyberShadow]

@CyberShadow can you please take a look at the failure?

It looks like compilation or execution of changed.d failed, but there is nothing in the log to indicate why. Possibly the process died because of a signal, e.g. due to running out of memory.