[Clang] Constexpr Structured Bindings : The easy parts (#160337)
This implements the easy parts of P2686R5.
Ie allowing constexpr structured binding of structs and arrays.
References to constexpr variables / support for tuple is left for a
future PR.
Until we implement the whole thing, the feature is not enabled as an
extension in older language modes.
Trying to use it as a tuple does produce errors but not meaningful ones.
We could add a better diagnostic if we fail to complete the
implementation before the end of the clang 22 cycle.