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

[Marco Elver via llvm-commits]

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+//===- AllocToken.h - Allocation token instrumentation --------------------===//
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melver wrote:

@ojhunt intended to build on it w/o the sanitizer terminology. And the builtin is usable without the "sanitizer mode" from https://github.com/llvm/llvm-project/pull/156842 onwards.

Actually, I started this as "SanitizeAllocPartition", but after this discussion here I renamed it: https://discourse.llvm.org/t/rfc-a-framework-for-allocator-partitioning-hints/87434/20?u=melver

I will defer on this.

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