Improve handling of floating point literals in OpenCL to only use double precision if the target supports fp64.
This change makes sure single-precision floating point types are used if the
cl_fp64 extension is not supported by the target.
Also removed the check to see whether the OpenCL version is >= 1.2, as this has
been incorporated into the extension setting code.
Differential Revision: https://reviews.llvm.org/D24235