[llvm] Add enum iteration to Sequence

This patch allows iterating typed enum via the ADT/Sequence utility.

It also changes the original design to better separate concerns:
 - `StrongInt` only deals with safe `intmax_t` operations,
 - `SafeIntIterator` presents the iterator and reverse iterator
 interface but only deals with safe `StrongInt` internally.
 - `iota_range` only deals with `SafeIntIterator` internally.

 This design ensures that operations are always valid. In particular,
 "Out of bounds" assertions fire when:
  - the `value_type` is not representable as an `intmax_t`
  - iterator operations make internal computation underflow/overflow
  - the internal representation cannot be converted back to `value_type`

Differential Revision: https://reviews.llvm.org/D106279