EIGEN_CONSTEXPR
Half
`bit_cast` cannot be `constexpr`, so we need to remove `EIGEN_CONSTEXPR` from `raw_half_as_uint16(...)`. This shouldn't affect anything else, since it is only used in `a bit_cast<uint16_t,half>()` which is not itself `constexpr`. Fixes #2077.
2ca1e73239