[OpenCL] Defines helper function for kernel language compatible OpenCL version

This change defines a helper function getOpenCLCompatibleVersion()
inside LangOptions class. The function contains mapping between
C++ for OpenCL versions and their corresponding compatible OpenCL
versions. This mapping function should be updated each time a new
C++ for OpenCL language version is introduced. The helper function
is expected to simplify conditions on OpenCL C and C++ for OpenCL
versions inside compiler code.

Code refactoring performed.

Differential Revision: https://reviews.llvm.org/D108693