[COFF] Fix crash when emitting symbol tables with GC

[smeenai]

Summary

When running with linker GC (-opt:ref), defined imported symbols that
are referenced but then dropped by GC end up with their Location
member being nullptr, which means getChunk() returns nullptr for them
and attempting to call getChunk()->getOutputSection() causes a crash
from the nullptr dereference. Check for getChunk() being nullptr and
bail out early to avoid the crash.

[ruiu]

LGTM

lld/trunk/COFF/Writer.cpp

case Symbol::DefinedAbsoluteKind:
     Sym.SectionNumber = IMAGE_SYM_ABSOLUTE;
     break;
   case Symbol::DefinedSyntheticKind:
     // Relative symbols are unrepresentable in a COFF symbol table.
     return None;
   default: {
     // Don't write symbols that won't be written to the output to the symbol
     // table.
-    OutputSection *OS = Def->getChunk()->getOutputSection();
+    Chunk *C = Def->getChunk();
+    if (!C)
+      return None;
+    OutputSection *OS = C->getOutputSection();
     if (!OS)
       return None;
 
     Sym.Value = Def->getRVA() - OS->getRVA();
     Sym.SectionNumber = OS->SectionIndex;
     break;
   }
   }

lld/trunk/test/COFF/symtab-gc.s

+# REQUIRES: x86
+
+# RUN: llvm-mc -filetype=obj -triple=x86_64-windows-msvc -o %tobject.obj %S/Inputs/object.s
+# RUN: lld-link -dll -entry:f -out:%t.dll -implib:%t.lib %tobject.obj
+# RUN: llvm-mc -filetype=obj -triple=x86_64-windows-msvc -o %tmain.obj %s
+# RUN: lld-link -entry:main -out:%t.exe -opt:ref -debug:dwarf %tmain.obj %t.lib
+# RUN: llvm-readobj -coff-imports %t.exe | FileCheck %s
+
+# CHECK-NOT: Symbol: f
+
+  .def   main;
+  .scl  2;
+  .type 32;
+  .endef
+  .section  .text,"xr",one_only,main
+  .globl  main
+main:
+  retq
+
+  .def   stripped;
+  .scl  3;
+  .type 32;
+  .endef
+  .section  .text,"xr",one_only,stripped
+stripped:
+  callq __imp_f
+  retq