[OpenCL] Change OpenCL builtin version encoding

Instead of using a MinVersion and MaxVersion field, encode the version
of a builtin using a mask that aligns better with version handling in
OpenCLOptions.h.  In addition, this saves a field in the BuiltinTable.

This change allows a finer-grained control over the OpenCL versions in
which a builtin is available: instead of a range, we can now toggle
each version individually.

The fine-grained version control is not yet exposed on the TableGen
definitions side, as changes for OpenCL 3 feature optionality still
need to be defined and will affect how we want to expose these.

Differential Revision: https://reviews.llvm.org/D100492