Show "LTO plugin missing" error correctly

Fixes #1403

Author: rui314

src/lto-unix.cc

@@ -598,6 +598,12 @@ create_plugin_input_file(Context<E> &ctx, MappedFile *mf) {
 
 template <typename E>
 ObjectFile<E> *read_lto_object(Context<E> &ctx, MappedFile *mf) {
+  if (ctx.arg.plugin.empty())
+    Fatal(ctx) << mf->name << ": don't know how to handle this LTO object file "
+               << "because no -plugin option was given. Please make sure you "
+               << "added -flto not only for creating object files but also for "
+               << "creating the final executable.";
+
   load_lto_plugin(ctx);
 
   // V0 API's claim_file is not thread-safe.
@@ -606,12 +612,6 @@ ObjectFile<E> *read_lto_object(Context<E> &ctx, MappedFile *mf) {
   if (!is_gcc_linker_api_v1)
     lock.lock();
 
-  if (ctx.arg.plugin.empty())
-    Fatal(ctx) << mf->name << ": don't know how to handle this LTO object file "
-               << "because no -plugin option was given. Please make sure you "
-               << "added -flto not only for creating object files but also for "
-               << "creating the final executable.";
-
   // Create mold's object instance
   ObjectFile<E> *obj = new ObjectFile<E>;
   ctx.obj_pool.emplace_back(obj);

test/lto-no-plugin.sh

@@ -0,0 +1,12 @@
+#!/bin/bash
+. $(dirname $0)/common.inc
+
+[ $MACHINE = $(uname -m) ] || skip
+
+echo 'int main() {}' | clang -B. -flto -o /dev/null -xc - >& /dev/null || skip
+
+echo 'int main() {}' | clang -c -o $t/a.o -xc -
+echo 'void foo() {}' | clang -c -o $t/b.o -xc - -flto
+
+! ./mold -o /dev/null $t/a.o $t/b.o >& $t/log
+grep -q "b.o: don't know how to handle this LTO object file" $t/log