[OpenCL] Supports optional 64-bit floating point types in C++ for OpenCL 2021

Adds support for a feature macro `__opencl_c_fp64` in C++ for OpenCL
2021 enabling a respective optional core feature from OpenCL 3.0.

This change aims to achieve compatibility between C++ for OpenCL
2021 and OpenCL 3.0.

Differential Revision: https://reviews.llvm.org/D108989