[COFF] clean up global resources after completion

[andrewrk]

Summary

Previously, running lld::coff::link() twice in the same process would access stale pointers because of these global variables not being reset. After this patch, lld::coff::link() can be called any number of times, just like its ELF and MACH-O counterparts.

[ruiu]

LGTM

[andrewrk]

Thanks. Can you do the commit? I do not have commit access.

COFF/Driver.cpp

bool link(ArrayRef<const char *> Args, bool CanExitEarly, raw_ostream &Diag) {
   Driver = make<LinkerDriver>();
   Driver->link(Args);
 
   // Call exit() if we can to avoid calling destructors.
   if (CanExitEarly)
     exitLld(errorCount() ? 1 : 0);
 
   freeArena();
+  ObjFile::Instances.clear();
+  ImportFile::Instances.clear();
+  BitcodeFile::Instances.clear();
   return !errorCount();
 }
 
 // Drop directory components and replace extension with ".exe" or ".dll".
 static std::string getOutputPath(StringRef Path) {
   auto P = Path.find_last_of("\\/");
   StringRef S = (P == StringRef::npos) ? Path : Path.substr(P + 1);
   const char* E = Config->DLL ? ".dll" : ".exe";