Fix OpenCL 1.2 double as an optional core feature behaviour

In OpenCL 1.2, using double no longer requires using the pragma cl_khr_fp64,
instead a kernel is allowed to use double, but must first have queried
clGetDeviceInfo's CL_DEVICE_DOUBLE_FP_CONFIG.

Page 197, section 6.1.1 of the OpenCL 1.2 specification has a footnote 23
describing this behaviour.

I've also added test cases such that the pragma must be used if targeting
OpenCL 1.0 or 1.1, but is ignored in 1.2 and 2.0.

Patch by Neil Henning!

Reviewers: Pekka Jääskeläinen

Differential Revision: http://reviews.llvm.org/D7245

llvm-svn: 227565