Inconsistency since `marray` is `constexpr` while `half` is not

According to the spec, the constructors of `marray` should be `constexpr`. Also, `marray` should accept `sycl::half` as element type (see 4.14.3 where it says "The element type parameter, DataT, must be a numeric type as it is defined by C++ standard."; `sycl::half` is a numeric type).

Hence, the following should compile
```
constexpr sycl::marray m{sycl::half{1}};
```
However, the spec does not require `half` to be constexpr-constructible and in C++17 this is basically impossible to achieve without using builtins that allow compile time bitcasts.

How should implementations handle this? Should there be a non-constexpr `marray` overload for `DataT == sycl::half`?

Review SYCL specification section 4.14.3 and the requirements for constexpr marray constructors, then compare them with the stated constraints on sycl::half in C++17. The issue is done when the specification clearly resolves whether marray must support constexpr construction with sycl::half or defines an alternative requirement.

Written by the indexing model from the issue text.