[LLD] COFF: Fix unsafe memory access

[ruiu]

Summary

BitcodeFile::M is freed before the writer writes a result because of presence
of BitcodeFile::releaseModule(). So we cannot keep pointers to bitcode files
in symbols.

[pcc]

LGTM

[Eugene.Zelenko]

Committed in r240046.

COFF/InputFiles.cpp

std::error_code BitcodeFile::parse() {
   M.reset(LTOModule::createFromBuffer(MB.getBufferStart(),
                                       MB.getBufferSize(),
                                       llvm::TargetOptions(), Err));
   if (!Err.empty()) {
     llvm::errs() << Err << '\n';
     return make_error_code(LLDError::BrokenFile);
   }
 
+  llvm::BumpPtrStringSaver Saver(Alloc);
   for (unsigned I = 0, E = M->getSymbolCount(); I != E; ++I) {
     lto_symbol_attributes Attrs = M->getSymbolAttributes(I);
     if ((Attrs & LTO_SYMBOL_SCOPE_MASK) == LTO_SYMBOL_SCOPE_INTERNAL)
       continue;
 
-    StringRef SymName = M->getSymbolName(I);
+    StringRef SymName = Saver.save(M->getSymbolName(I));
     int SymbolDef = Attrs & LTO_SYMBOL_DEFINITION_MASK;
     if (SymbolDef == LTO_SYMBOL_DEFINITION_UNDEFINED) {
       SymbolBodies.push_back(new (Alloc) Undefined(SymName));
     } else {
       bool Replaceable = (SymbolDef == LTO_SYMBOL_DEFINITION_TENTATIVE ||
                           (Attrs & LTO_SYMBOL_COMDAT));
       SymbolBodies.push_back(new (Alloc) DefinedBitcode(SymName, Replaceable));