[HLSL] Implement constant expression evaluator for HLSL splat cast

[spall]

See PR for Implementation of this cast #118992

Add support to handle the new cast in the constant expression evaluator (clang/lib/AST/ExprConstant.cpp)
Support to handle this cast should be added to:

• VectorExprEvaluator::VisitCastExpr
• RecordExprEvaluator::VisitCastExpr
• ArrayExprEvaluator::VisitCastExpr (There is currently no such function but I believe it should be added).

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Sarah Spall (spall)

See PR for Implementation of this cast #118992

Add support to handle the new cast in the constant expression evaluator (clang/lib/AST/ExprConstant.cpp)
Support to handle this cast should be added to:

• VectorExprEvaluator::VisitCastExpr
• RecordExprEvaluator::VisitCastExpr
• ArrayExprEvaluator::VisitCastExpr (There is currently no such function but I believe it should be added).