LLD COFF: close handles to objects used to link

fixes test failures on windows
See #302

patch sent upstream:
http://lists.llvm.org/pipermail/llvm-dev/2017-October/118234.html

deps/lld/COFF/Driver.cpp

@@ -61,6 +61,7 @@ bool link(ArrayRef<const char *> Args, raw_ostream &Diag) {
       (ErrorOS == &llvm::errs() && Process::StandardErrHasColors());
   Driver = make<LinkerDriver>();
   Driver->link(Args);
+  freeArena();
   return !ErrorCount;
 }