For the AllocToken pass to accurately calculate token ID hints, we
should attach !alloc_token_hint metadata for allocation calls to avoid
reverting to LLVM IR-type based hints (which depends on later "uses" and
is rather imprecise).

Unlike new expressions, untyped allocation calls (like malloc,
calloc, ::operator new(..), __builtin_operator_new, etc.) have no
syntactic type associated with them. For -fsanitize=alloc-token, type
hints are sufficient, and we can attempt to infer the type based on
common idioms.

When encountering allocation calls (with __attribute__((malloc)) or
__attribute__((alloc_size(..))), attach !alloc_token_hint by
inferring the allocated type from (a) sizeof argument expressions such
as malloc(sizeof(MyType)), and (b) casts such as (MyType*)malloc(4096).

Note that non-standard allocation functions with these attributes are
not instrumented by default. Use -fsanitize-alloc-token-extended to
instrument them as well.

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

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