GPU: Bump required LLVM version for OpenCL

Author: davidrohr

dependencies/FindO2GPU.cmake

@@ -175,7 +175,7 @@ if(ENABLE_OPENCL)
   if(Clang_FOUND
      AND LLVM_FOUND
      AND NOT LLVM_CLANG STREQUAL "LLVM_CLANG-NOTFOUND"
-     AND LLVM_PACKAGE_VERSION VERSION_GREATER_EQUAL 13.0)
+     AND LLVM_PACKAGE_VERSION VERSION_GREATER_EQUAL 18.0)
     set(OPENCL_COMPATIBLE_CLANG_FOUND ON)
   endif()
   if(OpenCL_VERSION_STRING VERSION_GREATER_EQUAL 2.2