[OpenCL] C++ for OpenCL version 2021 introduced to command line.

Introduces language standard `lang_openclcpp2021` and allows
`clc++2021` as a version flag for `-cl-std` in command line.
Defines macros related to C++ for OpenCL version 2021.

C++ for OpenCL version 2021 has been proposed in an RFC:
https://lists.llvm.org/pipermail/cfe-dev/2021-August/068593.html

Differential Revision: https://reviews.llvm.org/D108038