[OpenCL] Add cl_ext_float_atomics support (#359)

This backports https://reviews.llvm.org/D106343 and https://reviews.llvm.org/D109740.
KhronosGroup/SPIRV-LLVM-Translator#1440 was merged
into SPIRV, 0003-Support-cl_bf16_conversions.patch is not needed.

Signed-off-by: Haonan Yang <[email protected]>

Author: haonanya1