Stop using optimally-tiled images in external memory tests (#2349)

Support for optimally-tiled images and tiling inference in general is
implementation-defined. It should not be relied upon in the CTS.

Also build the code base as C++17 for std::optional. Many Khronos
projects are now using C++17 and the transition in the CTS is IMO
overdue.

Signed-off-by: Kevin Petit <kevin.petit@arm.com>

test_conformance/common/vulkan_wrapper/opencl_vulkan_wrapper.hpp

@@ -26,6 +26,8 @@
 #include <OpenCL/cl_ext.h>
 #endif
 
+#include <optional>
+
 #define CREATE_OPENCL_SEMAPHORE(clSemaphore, vkSemaphore, ctx, handleType,     \
                                 devIdx, createExportable)                      \
     if (!(createExportable                                                     \
@@ -187,8 +189,8 @@ public:
 extern void init_cl_vk_ext(cl_platform_id, cl_uint num_devices,
                            cl_device_id *deviceIds);
 
-VulkanImageTiling vkClExternalMemoryHandleTilingAssumption(
+std::optional<VulkanImageTiling> vkClExternalMemoryHandleTilingAssumption(
     cl_device_id deviceId,
     VulkanExternalMemoryHandleType vkExternalMemoryHandleType, int *error_ret);
 
-#endif // _opencl_vulkan_wrapper_hpp_
+#endif // _opencl_vulkan_wrapper_hpp_