[OpenCL] Simplify InsertOCLBuiltinDeclarationsFromTable

 - Use a range-based for loop.  This will help a later patch to skip
   prototypes that use an unavailable return type or argument type.

 - Replace a dyn_cast with a cast, as we are only dealing with
   FunctionProtoType Types here.

GitOrigin-RevId: af67deb005cae4e87109d8ad534446b300351483
1 file changed