[ELF] --warn-backrefs: --defsym does not make a backward reference

The interaction between --warn-backrefs was not tested, but if
--defsym-created reference causes archive member extraction, it seems
reasonable to suppress the diagnostic, which was the behavior before #78944.

(cherry picked from commit 9a1ca24)

Author: MaskRay

lld/ELF/InputFiles.cpp

@@ -1788,7 +1788,12 @@ void BinaryFile::parse() {
 }
 
 InputFile *elf::createInternalFile(StringRef name) {
-  return make<InputFile>(InputFile::InternalKind, MemoryBufferRef("", name));
+  auto *file =
+      make<InputFile>(InputFile::InternalKind, MemoryBufferRef("", name));
+  // References from an internal file do not lead to --warn-backrefs
+  // diagnostics.
+  file->groupId = 0;
+  return file;
 }
 
 ELFFileBase *elf::createObjFile(MemoryBufferRef mb, StringRef archiveName,

lld/test/ELF/warn-backrefs.s

@@ -100,6 +100,10 @@
 ## -u does not make a backward reference.
 # RUN: ld.lld --fatal-warnings --warn-backrefs -u foo %t2.a %t1.o -o /dev/null
 
+## --defsym does not make a backward reference, but it does not suppress the warning due to another file.
+# RUN: ld.lld --fatal-warnings --warn-backrefs --defsym=x=foo -e 0 %t2.a -o /dev/null
+# RUN: ld.lld --warn-backrefs --defsym=x=foo %t2.a %t1.o -o /dev/null 2>&1 | FileCheck %s
+
 # RUN: not ld.lld --warn-backrefs-exclude='[' 2>&1 | FileCheck --check-prefix=INVALID %s
 # INVALID: error: --warn-backrefs-exclude: invalid glob pattern, unmatched '[': [