add checks for OpenCL C versions (#968)

* add test_compiler test for OpenCL C versions

* add checks for required OpenCL C versions

* bugfix: only OpenCL C 2.0 is required for OpenCL 2.1 and 2.2

* pass reference to feature struct vs. pointer

* address review comments regarding C++ identifiers and 3d image writes

test_conformance/compiler/CMakeLists.txt

@@ -7,6 +7,7 @@ set(${MODULE_NAME}_SOURCES
     test_async_build.cpp
     test_build_options.cpp
     test_preprocessor.cpp
+    test_opencl_c_versions.cpp
     test_image_macro.cpp
     test_compiler_defines_for_extensions.cpp
     test_pragma_unroll.cpp