[OpenCL patch] support for __builtin_astype, __builtin_convert, __builtin_vec_step

Hi Tanya,

Similar to Guy, I'm wondering what's the benefit of representing
as/convert_type() as nodes in the AST. To me, they are just like other
built-in functions. [Indeed, convert_type() implementations, especially for
different rounding and saturation modes, can be quite involved and
target-specific.]

If we add special support for as/convert_type() to Clang, don't we need to
add special support for all the other built-in functions as well? (I hope
not, as we are very sensitive to the size of Clang.)

Best regards,
Anton.