ld: Keep indirect symbol from IR if referenced from shared object

Don't change indirect symbol defined in IR to undefined if it is
referenced from shared object.

bfd/

	PR ld/28879
	* elflink.c (_bfd_elf_merge_symbol): Don't change indirect
	symbol defined in IR to undefined if it is referenced from
	shared object.

ld/

	PR ld/28879
	* testsuite/ld-plugin/lto.exp: Run PR ld/28879 tests.
	* testsuite/ld-plugin/pr28879a.cc: New file.
	* testsuite/ld-plugin/pr28879b.cc: Likewise.

bfd/elflink.c

@@ -1294,9 +1294,8 @@ _bfd_elf_merge_symbol (bfd *abfd,
 	  h->root.non_ir_ref_dynamic = true;
 	  hi->root.non_ir_ref_dynamic = true;
 	}
-
-      if ((oldbfd->flags & BFD_PLUGIN) != 0
-	  && hi->root.type == bfd_link_hash_indirect)
+      else if ((oldbfd->flags & BFD_PLUGIN) != 0
+	       && hi->root.type == bfd_link_hash_indirect)
 	{
 	  /* Change indirect symbol from IR to undefined.  */
 	  hi->root.type = bfd_link_hash_undefined;

ld/testsuite/ld-plugin/lto.exp

@@ -471,6 +471,32 @@ set lto_link_elf_tests [list \
   [list {pr27441c.so} \
    {-shared -fPIC -Wl,--as-needed tmpdir/pr27441c.o tmpdir/pr27441b.so tmpdir/pr27441a.so} {-fPIC} \
    {dummy.c} {{readelf {-dW} pr27441c.d}} {pr27441c.so}] \
+  [list \
+   "Build libpr28879a.so" \
+   "-shared" \
+   "-O0 -fpic" \
+   {pr28879a.cc} \
+   {} \
+   "libpr28879a.so" \
+   "c++" \
+  ] \
+  [list \
+   "Build libpr28879b.so" \
+   "-shared -Wl,--no-as-needed tmpdir/libpr28879a.so" \
+   "-O2 -fpic" \
+   {dummy.c} \
+   {} \
+   "libpr28879b.so" \
+  ] \
+  [list \
+   "Build pr28879" \
+   "-Wl,--no-as-needed tmpdir/libpr28879b.so -Wl,-rpath-link,." \
+   "-O0 -flto -D_GLIBCXX_ASSERTIONS" \
+   {pr28879b.cc} \
+   {} \
+   "pr28879" \
+   "c++" \
+  ] \
 ]
 
 # PR 14918 checks that libgcc is not spuriously included in a shared link of

ld/testsuite/ld-plugin/pr28879a.cc

@@ -0,0 +1,7 @@
+#include <string>
+
+void
+func (std::string *s)
+{
+  delete s;
+}

ld/testsuite/ld-plugin/pr28879b.cc

@@ -0,0 +1,8 @@
+#include <string>
+
+int
+main (void)
+{
+  std::string header;
+  return 0;
+}