llvm: Ignore error value that is always false

See llvm/llvm-project#121851

For LLVM 20+, this function (`renameModuleForThinLTO`) has no return
value. For prior versions of LLVM, this never failed, but had a
signature which allowed an error value people were handling.

src/back/lto.rs

@@ -660,9 +660,7 @@ pub unsafe fn optimize_thin_module(
         {
             let _timer =
                 cgcx.prof.generic_activity_with_arg("LLVM_thin_lto_rename", thin_module.name());
-            if !llvm::LLVMRustPrepareThinLTORename(thin_module.shared.data.0, llmod, target) {
-                return Err(write::llvm_err(&dcx, LlvmError::PrepareThinLtoModule));
-            }
+            unsafe { llvm::LLVMRustPrepareThinLTORename(thin_module.shared.data.0, llmod, target) };
             save_temp_bitcode(cgcx, &module, "thin-lto-after-rename");
         }