[llvm] [AllocToken] Introduce AllocToken instrumentation pass (PR #156838)

[Marco Elver via llvm-commits]

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@@ -2427,6 +2427,9 @@ For example:
     if the attributed function is called during invocation of a function
     attributed with ``sanitize_realtime``.
     This attribute is incompatible with the ``sanitize_realtime`` attribute.
+``sanitize_alloc_token``
+    This attributes indicates that implicit allocation token instrumentation
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melver wrote:

I think historically it was to avoid accidental instrumentation of e.g. generated functions.

But in our case it's deliberate, and it's also the only reasonable way to then proceed towards making it process the builtin (and potentially what @ojhunt wants to do with TMO) unconditionally: https://github.com/llvm/llvm-project/pull/156842

https://github.com/llvm/llvm-project/pull/156838