IsMissing should be true, not false; NFC

This is an NFC change because that parameter is unused by the
DependencyCollector currently; there's no way to test for the change.

Author: AaronBallman

clang/lib/Frontend/DependencyFile.cpp

@@ -81,7 +81,7 @@ struct DepCollectorPPCallbacks : public PPCallbacks {
         /*FromModule*/ false,
         /*IsSystem*/ false,
         /*IsModuleFile*/ false,
-        /*IsMissing*/ false);
+        /*IsMissing*/ true);
     // Return true to silence the file not found diagnostic.
     return true;
   }