[libc++] Fix constraints for `optional`'s constructors taking `in_place_t` and related `make_optional` overloads (#173467) Some constraints are incorrect for constructors of `optional<T&>`: - for the `(in_place_t, Arg&&)` constructor, it should be more constrained to reject dangling references; - for the `(in_place_t, initializer_list<U>, Args&&...)` constructor, it shouldn't be available for `optional<T&>` at all. For `make_optional` overloads, the standard wording already required them to propagate SFINAE constraints before LWG3627 (via "_Effects_: Equivalent to", see also [structure.specifications]/4). So they need to be constrained. Drive-by: Refactor test files to run more cases during constant evaluation.
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