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

[Hans Wennborg 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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zmodem wrote:

(ultra nit: no s on "attributes")

I noticed that some attributes seem opt-in, like this and `sanitize_address`, while some are opt-out like `nosanitize_bounds`. I wonder if those were deliberate choices or just happened that way.

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