Refactor the AllocToken pass to accept the mode via pass options rather
than LLVM cl::opt. This is both cleaner, but also required to make the
mode frontend-driven and avoid potential inconsistencies.
Refactor the stateless (hash-based) token calculation logic out of the
`AllocToken` pass and into `llvm/Support/AllocToken.h`.
This helps with making the token calculation logic available to other
parts of the codebase, which will be necessary for frontend
implementation of `__builtin_infer_alloc_token` to perform constexpr
evaluation.
The `AllocTokenMode` enum and a new `AllocTokenMetadata` struct are
moved into a shared header. The `getAllocTokenHash()` function now
provides the source of truth for calculating token IDs for `TypeHash`
and `TypeHashPointerSplit` modes.
