[llvm-objcopy] Strip debug sections when running with --strip-unneeded.

[rupprecht]

Summary

GNU --strip-unneeded strips debugging sections as well. Do that for llvm-objcopy as well.

Additionally, add a test that verifies we keep the .gnu_debuglink section. This apparently was not always the case, and I'm not sure which commit fixed it, but there doesn't appear to be any test coverage to make sure we continue to do so.

This fixes PR41043.

[jhenderson]

LGTM.

llvm/test/tools/llvm-objcopy/ELF/strip-unneeded-remove-debug-keep-link.test

+## Verifies that running with --strip-unneeded removes debugging sections but
+## retains the .gnu_debuglink section.
+
+# RUN: yaml2obj %s > %t.o
+# RUN: touch %t.debug
+# RUN: llvm-objcopy --add-gnu-debuglink=%t.debug %t.o %t.2.o
+# RUN: llvm-strip --strip-unneeded %t.2.o -o %t.3.o
+# RUN: llvm-readelf --sections %t.3.o | FileCheck %s
+
+!ELF
+FileHeader:
+  Class:           ELFCLASS64
+  Data:            ELFDATA2LSB
+  Type:            ET_REL
+  Machine:         EM_X86_64
+Sections:
+  - Name:            .text
+    Type:            SHT_PROGBITS
+    Flags:           [ SHF_ALLOC, SHF_EXECINSTR ]
+  - Name:            .debugfoo
+    Type:            SHT_PROGBITS
+
+# CHECK: There are 6 section headers
+
+# CHECK:        [ 0]
+# CHECK-NEXT:   [ 1] .text
+# CHECK-NEXT:   [ 2] .symtab
+# CHECK-NEXT:   [ 3] .strtab
+# CHECK-NEXT:   [ 4] .shstrtab
+# CHECK-NEXT:   [ 5] .gnu_debuglink

llvm/tools/llvm-objcopy/ELF/ELFObjcopy.cpp

if (Config.StripAllGNU)
     };
 
   if (Config.StripSections) {
     RemovePred = [RemovePred](const SectionBase &Sec) {
       return RemovePred(Sec) || Sec.ParentSegment == nullptr;
     };
   }
 
-  if (Config.StripDebug) {
+  if (Config.StripDebug || Config.StripUnneeded) {
     RemovePred = [RemovePred](const SectionBase &Sec) {
       return RemovePred(Sec) || isDebugSection(Sec);
     };
   }
 
   if (Config.StripNonAlloc)
     RemovePred = [RemovePred, &Obj](const SectionBase &Sec) {
       if (RemovePred(Sec))