Introduce AllocToken, an instrumentation pass designed to provide
tokens to memory allocators enabling various heap organization
strategies, such as heap partitioning.

Initially, the pass instruments functions marked with a new attribute
sanitize_alloc_token by rewriting allocation calls to include a token
ID, appended as a function argument with the default ABI.

The design aims to provide a flexible framework for implementing
different token generation schemes. It currently supports the following
token modes:

• TypeHash (default): token IDs based on a hash of the allocated type
• Random: statically-assigned pseudo-random token IDs
• Increment: incrementing token IDs per TU

For the TypeHash mode introduce support for !alloc_token_hint
metadata: the metadata can be attached to allocation calls to provide
richer semantic information to be consumed by the AllocToken pass.
Optimization remarks can be enabled to show where no metadata was
available.

An alternative "fast ABI" is provided, where instead of passing the
token ID as an argument (e.g., __alloc_token_malloc(size, id)), the
token ID is directly encoded into the name of the called function (e.g.,
__alloc_token_0_malloc(size)). Where the maximum tokens is small, this
offers more efficient instrumentation by avoiding the overhead of
passing an additional argument at each allocation site.

Link: https://discourse.llvm.org/t/rfc-a-framework-for-allocator-partitioning-hints/87434 [1]

This change is part of the following series:

1. [AllocToken] Introduce AllocToken instrumentation pass #156838
2. [Clang] Introduce -fsanitize=alloc-token #156839
3. [AllocToken, Clang] Implement TypeHashPointerSplit mode #156840
4. [AllocToken, Clang] Infer type hints from sizeof expressions and casts #156841
5. [AllocToken, Clang] Implement __builtin_alloc_token_infer() and llvm.alloc.token.id #156842