[ELF] - Fix use of freed memory.

[grimar]

Summary

It was revealed by D27831.

If we have linkerscript that includes another one that sets OUTPUT for example:

1. RUN: echo "INCLUDE \"foo.script\"" > %t.script
2. RUN: echo "OUTPUT(\"%t.out\")" > %T/foo.script

then we do:

void ScriptParser::readInclude() {
...
  std::unique_ptr<MemoryBuffer> &MB = *MBOrErr;
  tokenize(MB->getMemBufferRef());
  OwningMBs.push_back(std::move(MB));
}

void ScriptParser::readOutput() {
...
    Config->OutputFile = unquote(Tok);
...
}

Problem is that OwningMBs are destroyed after script parser do its job.
So all Toks are dead and Config->OutputFile points to destroyed data.

Patch suggests to save all included scripts into using string Saver.

[ruiu]

LGTM

ELF/LinkerScript.cpp

private:
   // For parsing version script.
   std::vector<SymbolVersion> readVersionExtern();
   void readAnonymousDeclaration();
   void readVersionDeclaration(StringRef VerStr);
   std::vector<SymbolVersion> readSymbols();
 
   ScriptConfiguration &Opt = *ScriptConfig;
   bool IsUnderSysroot;
-  std::vector<std::unique_ptr<MemoryBuffer>> OwningMBs;
 };
 
 void ScriptParser::readDynamicList() {
   expect("{");
   readAnonymousDeclaration();
   if (!atEOF())
     setError("EOF expected, but got " + next());
 }

 void ScriptParser::readInclude() {
   StringRef Tok = next();
   auto MBOrErr = MemoryBuffer::getFile(unquote(Tok));
   if (!MBOrErr) {
     setError("cannot open " + Tok);
     return;
   }
   std::unique_ptr<MemoryBuffer> &MB = *MBOrErr;
-  tokenize(MB->getMemBufferRef());
-  OwningMBs.push_back(std::move(MB));
+  tokenize({Saver.save(MB->getBuffer()), unquote(Tok)});
 }
 
 void ScriptParser::readOutput() {
   // -o <file> takes predecence over OUTPUT(<file>).
   expect("(");
   StringRef Tok = next();
   if (Config->OutputFile.empty())
     Config->OutputFile = unquote(Tok);