fix: clover: LLVM 18: s/CodeGenOpt::/CodeGenOptLevel::/

Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/9833
Reference: https://github.com/llvm/llvm-project/commit/0a1aa6cda2758b0926a95f87d39ffefb1cb90200
Signed-off-by: Kai Wasserbäch <kai@dev.carbon-project.org>
Reviewed-by: Karol Herbst <kherbst@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/25586>

src/gallium/frontends/clover/llvm/codegen/native.cpp

@@ -121,7 +121,11 @@ namespace {
 #else
                                 ::llvm::None, ::llvm::None,
 #endif
+#if LLVM_VERSION_MAJOR >= 18
+                                ::llvm::CodeGenOptLevel::Default) };
+#else
                                 ::llvm::CodeGenOpt::Default) };
+#endif
       if (!tm)
          fail(r_log, build_error(),
               "Could not create TargetMachine: " + target.triple);