[OpenCL] Add version handling and add vector ld/st builtins

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.

Differential Revision: https://reviews.llvm.org/D63504

llvm-svn: 372321