Allow setting a MinVersion, stating from which OpenCL version a
builtin function is available, and a MaxVersion, stating from which
OpenCL version a builtin function should not be available anymore.
Guard some definitions of the "work-item" builtin functions according
to the OpenCL versions from which they are available.
Add the "vector data load and store" builtin functions (e.g. vload/vstore),
whose signatures differ before and after OpenCL 2.0 in the pointer
argument address spaces.
Patch by Pierre Gondois and Sven van Haastregt.