Support for cl_ext_cxx_for_opencl

[MathiasMagnus]

Support for online compilation of C++ for OpenCL via exposing the cl_ext_cxx_for_opencl extension. Bundling the necessary parts of the Clang/LLVM/LLVM-SPIRV projects as a library and triggering when the user feeds „-cl-std=clc++” to clBuildProgram() and doing a roundtrip to spirv2clc instead of generating vendor IR/bytecode directly.