[HLSL] Use consistent style for parameter names for built-in methods

We do not have a consistent naming style for parameter names of built-in object methods that are created in `HLSLBuiltinTypeDeclBuilder`. Some parameters are lowercase, others have their first name letter capitalized. We should decide on a consistent naming scheme a stick to it.

See
- https://github.com/llvm/llvm-project/blob/3fcba45de123f2c3a09faf50b51f507437fe6b21/clang/lib/Sema/HLSLBuiltinTypeDeclBuilder.cpp#L1590
- https://github.com/llvm/llvm-project/blob/3fcba45de123f2c3a09faf50b51f507437fe6b21/clang/lib/Sema/HLSLBuiltinTypeDeclBuilder.cpp#L1325
- https://github.com/llvm/llvm-project/blob/3fcba45de123f2c3a09faf50b51f507437fe6b21/clang/lib/Sema/HLSLBuiltinTypeDeclBuilder.cpp#L1037
- https://github.com/llvm/llvm-project/blob/3fcba45de123f2c3a09faf50b51f507437fe6b21/clang/lib/Sema/HLSLBuiltinTypeDeclBuilder.cpp#L1533