Extended list of valid frontend options with '-cl-std=CL2.0'.

This option sets language mode for the compilation of a source file to be OpenCL v2.0.

Example: clang -cc1 -cl-std=CL2.0 myfile.cl



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@222444 91177308-0d34-0410-b5e6-96231b3b80d8