[HLSL] Cleanup TargetInfo handling (#90694)

We had some odd places where we set target behaviors. We were setting
the long size in target-specific code, but it should be language-based.
We were not setting the Half float type semantics correctly, and instead
were overriding the query in the AST context.

This change it moves existing code to the right places in the Target so
that as we continue working on target and language feature they are
controlled in the right places.

It also fixes a bug where the size of `half` was computed incorrectly
when native half types are not supported.