[HLSL] Add support to constant expression evaluator for 'IntegralToFloating' and other similar casts to VectorExprEvaluator

[spall]

HLSL allows IntegeralToFloating and other similar casts to be applied to vectors. Support for the following casts should be added to VectorExprEvaluator::VisitCastExpr.

• IntegralToFloating
• FloatingToIntegral
• IntegralCast
• FloatingCast
• FloatingToBoolean
• IntegralToBoolean

https://godbolt.org/z/n6P9E3czq

Tests should be added showing these casts being successfully performed in 'constexpr' on vectors.

[damyanp]

Although this could be viewed as being required for completeness of implementing these casts, we're going to think of this as more a part of implementing constexpr for HLSL.