Remove reference to `mlir::Operations` in `Thunks`.
Remove reference to `mlir::Operations` in `Thunks`.

The `mlir::Operation` in `Thunk` (`Thunk::op_`) is never set to a value different from `nullptr`.

Conveniently, this also removes the need for `Thunk::ClearCompileTimeInfo`, which in turn leaves `ForAllThunks` (in `compile_module_to_llvm_ir.cc`) unused.

I'm doing this first so that I can then (a) move `ForAllThunks` to its own file, and (b) change its behavior to actually iterate over all `Thunks`: for example, `ForAllThunks` currently does not iterate over the `embedded_thunk` in `AddressComputationThunk`. This behavior will be safer to change after there's no remaining users of the function.
