[clang] [AllocToken] Enable alloc token instrumentation for size-returning functions (PR #168840)

[Marco Elver via cfe-commits]

https://github.com/melver approved this pull request.

LGTM

Re commit desc: Not only `__size_returning_new` (all decls will need the attribute to work, it won't work automatically), but all functions where we add the attribute.

With `-fsanitize-alloc-token-extended` it'll also work for custom non-standard allocations functions that add the attribute.

https://github.com/llvm/llvm-project/pull/168840