[Clang] Move AllocToken frontend options to LangOptions (#163635)
Move the `AllocTokenMax` from `CodeGenOptions` and introduces a new
`AllocTokenMode` to `LangOptions`. Note, `-falloc-token-mode=`
deliberately remains an internal experimental option.
This refactoring is necessary because these options influence frontend
behavior, specifically constexpr evaluation of `__builtin_infer_alloc_token`.
Placing them in `LangOptions` makes them accessible during semantic analysis,
which occurs before codegen.