[HLSL] Make casting functions constexpr (#108902)
This marks the `bit_cast` helper as `constexpr` and allows the casts
implemented with it to also be `constexpr`. This is largely not a
functional change, but it enables using the casts in expressions that
need to be resolved at compile time as demonstrated with the static
asserts in the new tests.
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Co-authored-by: joaosaffran <126493771+joaosaffran@users.noreply.github.com>
