Driver: refactor OpenCL argument forwarding

Extract the argument forwarding for OpenCL arguments.  Make this more
data driven as we are just repeating the argument name and spelling.
This costs a slight bit more memory due to the string duplication, but
makes it easier to follow.  It should be possible to forward the
internal string representation from the TableGen data to avoid this.
But, this makes the code simpler to follow for now.

llvm-svn: 312083