[ELF] Don't error on relocations to STT_SECTION symbols if the sections were discarded

[MaskRay]

Summary

This essentially reverts r362743, which restored r361830.

Reland D61583 [ELF] Error on relocations to STT_SECTION symbols if the sections were discarded

The reverts of this revision are tangled so it is a bit tricky to revert it.
This change keeps the goodness of D61583 while fixing the line table issue as reported in
https://bugs.chromium.org/p/chromium/issues/detail?id=978067

[hans]

The reverts of this revision are tangled so it is a bit tricky to revert it.

I reverted already in r362743.

ELF/InputFiles.cpp

if (Binding == STB_LOCAL) {
       if (ESym.getType() == STT_FILE)
         SourceFile = CHECK(ESym.getName(this->StringTable), this);
 
       if (this->StringTable.size() <= ESym.st_name)
         fatal(toString(this) + ": invalid symbol name offset");
 
       if (ESym.st_shndx == SHN_UNDEF)
         this->Symbols[I] = make<Undefined>(this, Name, Binding, StOther, Type);
-      else if (Sec == &InputSection::Discarded)
-        this->Symbols[I] = make<Undefined>(this, Name, Binding, StOther, Type,
-                                           /*DiscardedSecIdx=*/SecIdx);
       else
         this->Symbols[I] =
             make<Defined>(this, Name, Binding, StOther, Type, Value, Size, Sec);
       continue;
     }
 
     // Handle global undefined symbols.
     if (ESym.st_shndx == SHN_UNDEF) {

ELF/Relocations.cpp

 template <class ELFT>
 static std::string maybeReportDiscarded(Undefined &Sym) {
   auto *File = dyn_cast_or_null<ObjFile<ELFT>>(Sym.File);
   if (!File || !Sym.DiscardedSecIdx ||
       File->getSections()[Sym.DiscardedSecIdx] != &InputSection::Discarded)
     return "";
   ArrayRef<Elf_Shdr_Impl<ELFT>> ObjSections =
       CHECK(File->getObj().sections(), File);
-
-  std::string Msg;
-  if (Sym.Type == ELF::STT_SECTION) {
-    Msg = "relocation refers to a discarded section: ";
-    Msg += CHECK(
-        File->getObj().getSectionName(&ObjSections[Sym.DiscardedSecIdx]), File);
-  } else {
-    Msg = "relocation refers to a symbol in a discarded section: " +
-          toString(Sym);
-  }
-  Msg += "\n>>> defined in " + toString(File);
+  std::string Msg =
+      "relocation refers to a symbol in a discarded section: " + toString(Sym) +
+      "\n>>> defined in " + toString(File);
 
   Elf_Shdr_Impl<ELFT> ELFSec = ObjSections[Sym.DiscardedSecIdx - 1];
   if (ELFSec.sh_type != SHT_GROUP)
     return Msg;
 
   // If the discarded section is a COMDAT.
   StringRef Signature = File->getShtGroupSignature(ObjSections, ELFSec);
   if (const InputFile *Prevailing =

test/ELF/comdat-discarded-error.s

 # REQUIRES: x86
+# XFAIL: *
 # RUN: llvm-mc -filetype=obj -triple=x86_64 %s -o %t1.o
 # RUN: echo '.section .text.foo,"axG",@progbits,foo,comdat; .globl foo; foo:' |\
 # RUN:   llvm-mc -filetype=obj -triple=x86_64 - -o %t2.o
 # RUN: echo '.section .text.foo,"axG",@progbits,foo,comdat; .globl bar; bar:' |\
 # RUN:   llvm-mc -filetype=obj -triple=x86_64 - -o %t3.o
 
 # RUN: not ld.lld %t2.o %t3.o %t1.o -o /dev/null 2>&1 | FileCheck %s
 

test/ELF/comdat-discarded-reloc.s

 # REQUIRES: x86
 # RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t.o
 # RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %p/Inputs/comdat-discarded-reloc.s -o %t2.o
-# RUN: ld.lld -gc-sections --noinhibit-exec %t2.o %t.o -o /dev/null
-# RUN: ld.lld -r %t2.o %t.o -o %t 2>&1 | FileCheck --check-prefix=WARN %s
-# RUN: llvm-readobj -r %t | FileCheck --check-prefix=RELOC %s
+# RUN: ld.lld -gc-sections %t.o %t2.o -o %t
 
 ## ELF spec doesn't allow a relocation to point to a deduplicated
 ## COMDAT section. Unfortunately this happens in practice (e.g. .eh_frame)
 ## Test case checks we do not crash.
 
 # WARN: warning: relocation refers to a discarded section: .text.bar1
 # WARN-NEXT: >>> referenced by {{.*}}.o:(.rela.text.bar2+0x0)
 # WARN-NOT: warning