[libc++] Resolve LWG4308, correct `iterator` availability for `optional<T&>` (#173948) Resolves #171345 Implements [proposed resolution for LWG4308](https://cplusplus.github.io/LWG/issue4308) and removes `const_iterator` from `optional<T&>`, which was missed. - Constrains iterator to only be available if T is not an lvalue reference, or if it is T&, that T is an object type and is not an unbounded array - Add a partial specialization for `__optional_iterator` for `T&`, which only has the `iterator` type. - Correct a static assert message as a drive-by - Move the libcxx specific iterator test into the standard test because the standard now specifies when the iterator should be available
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