Refactor the logic for inferring allocated types out of `CodeGen` and
into a new reusable component in `clang/AST/InferAlloc.h`.
This is a preparatory step for implementing `__builtin_infer_alloc_token`.
By moving the type inference heuristics into a common place, it can be
shared between the existing allocation-call instrumentation and the new
builtin's implementation.
