[HLSL] Expose `half` types and intrinsics always (#81782)

We previously made an implmenetation error when adding `half` overloads
for HLSL library functionalitly. The `half` type is always defined in
HLSL and `half` intrinsics should not be conditionally included.

When native 16-bit types are disabled `half` is a unique 32-bit float
type with lesser promotion rank than `float`.

Fixes #81049