[OpenCL] An implementation of device side enqueue (DSE) from OpenCL v2.0 s6.13.17.

- Added new Builtins: enqueue_kernel, get_kernel_work_group_size
and get_kernel_preferred_work_group_size_multiple.

These Builtins use custom check to diagnose parameters of the passed Blocks
i. e. variable number of 'local void*' type params, and check different
overloads specified in Table 6.31 of OpenCL v2.0.

- IR is generated as an internal library call for each OpenCL Builtin,
reusing ObjC Block implementation.

Review: http://reviews.llvm.org/D20249
llvm-svn: 274540