[Driver] For the persistent PCH job there is no output path, the frontend determines the filename to use.

Previously the parseable output would have an empty string as the output of the PCH job, this fixes the issue.

lib/Driver/Driver.cpp

@@ -1759,8 +1759,7 @@ static StringRef getOutputFilename(Compilation &C,
   if (auto *PCHAct = dyn_cast<GeneratePCHJobAction>(JA)) {
     // For a persistent PCH we don't use an output, the frontend determines
     // the filename to use for the PCH.
-    if (PCHAct->isPersistentPCH())
-      return StringRef();
+    assert(!PCHAct->isPersistentPCH());
     AtTopLevel = false;
   }