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



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@289544 91177308-0d34-0410-b5e6-96231b3b80d8