Fix spurious "dupilcate symbol" error for LTO

Fixes #1421

Author: rui314

src/passes.cc

@@ -1036,6 +1036,9 @@ void check_duplicate_symbols(Context<E> &ctx) {
   Timer t(ctx, "check_duplicate_symbols");
 
   tbb::parallel_for_each(ctx.objs, [&](ObjectFile<E> *file) {
+    if (!file->is_reachable)
+      return;
+
     for (i64 i = file->first_global; i < file->elf_syms.size(); i++) {
       const ElfSym<E> &esym = file->elf_syms[i];
       Symbol<E> &sym = *file->symbols[i];

test/lto-archive3.sh

@@ -0,0 +1,19 @@
+#!/bin/bash
+. $(dirname $0)/common.inc
+
+test_cflags -flto=auto || skip
+
+cat <<EOF | $CC -o $t/a.o -c -xc - -flto
+void foo() {}
+EOF
+
+rm -f $t/b.a
+ar rc $t/b.a $t/a.o
+ar rc $t/c.a $t/a.o
+
+cat <<EOF | $CC -o $t/c.o -c -xc -
+void foo();
+int main() { foo(); }
+EOF
+
+$CC -B. -o $t/exe -flto $t/c.o $t/b.a $t/c.a