[ADT] Extend EnumeratedArray

EnumeratedArray is essentially a wrapper around a fixed-size
array that uses enum values instead of integers as indices.

 * Add iterator support (begin/end/rbegin/rend), which enables
   the use of iterator/range based algorithms on EnumeratedArrays.
 * Add common container typedefs (value_type etc.), allowing
   drop-in replacements of other containers in cases relying on these.
 * Add a constructor that takes an std::initializer_list<T>.
 * Make the size() function const.
 * Add empty().

Iterator support slightly lowers the protection non-type-safe accesses,
because iterator arithmetic is not enum-based, and one can now use
*(begin() + IntIndex). However, it is and was also always possible to
just cast arbitrary indices to the enum type.

Differential Revision: https://reviews.llvm.org/D135594

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