ELF: Do not follow relocation edges to output sections during GC.

[pcc]

Summary

This fixes an assertion error introduced by r320390.

lld/ELF/MarkLive.cpp

static void resolveReloc(InputSectionBase &Sec, RelT &Rel,
 
   // If a symbol is referenced in a live section, it is used.
   B.Used = true;
   if (auto *SS = dyn_cast<SharedSymbol>(&B))
     if (!SS->isWeak())
       SS->getFile<ELFT>()->IsNeeded = true;
 
   if (auto *D = dyn_cast<Defined>(&B)) {
-    if (!D->Section)
+    auto *RelSec = dyn_cast_or_null<InputSectionBase>(D->Section);
+    if (!RelSec)
       return;
     uint64_t Offset = D->Value;
     if (D->isSection())
       Offset += getAddend<ELFT>(Sec, Rel);
-    Fn(cast<InputSectionBase>(D->Section), Offset);
+    Fn(RelSec, Offset);
     return;
   }
 
   if (!B.isDefined())
     for (InputSectionBase *Sec : CNamedSections.lookup(B.getName()))
       Fn(Sec, 0);
 }
 

lld/test/ELF/gc-sections-linker-defined-symbol.s

 # REQUIRES: x86
 
 # RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t.o
 # RUN: ld.lld %t.o -o %t.so --gc-sections -shared
 # RUN: llvm-readobj --dyn-symbols %t.so | FileCheck %s
 
 # CHECK:      Name: _end@
 # CHECK-NEXT: Value:
 # CHECK-NEXT: Size:
 # CHECK-NEXT: Binding: Global
 # CHECK-NEXT: Type: None
 # CHECK-NEXT: Other:
 # CHECK-NEXT: Section: .dynamic
 
         .data
+        .globl g
+        g:
         .quad _end