constants: make to_string handle more values

This is mostly implemented for `clGetKernelArgInfo` [1] called with
`CL_KERNEL_ARG_TYPE_QUALIFIER`. Since an argument can have multiple
type qualifiers (e.g. `const` and `volatile`), this returns the bitwise
or of the flags that we need to separate.

[1] https://www.khronos.org/registry/OpenCL/sdk/1.2/docs/man/xhtml/clGetKernelArgInfo.html