Make global symbols survive GC runs

- The global symbol table was never marked by the GC, so anything pointed
  only by it was fair game to disappear.
- Don't clear the global symbol table during DECORATE parsing. Junk in
  there should be considered constant after initialization.

src/thingdef/thingdef.cpp

@@ -425,7 +425,6 @@ void LoadActors ()
 
 	timer.Reset(); timer.Clock();
 	ActorDamageFuncs.Clear();
-	GlobalSymbols.ReleaseSymbols();
 	FScriptPosition::ResetErrorCounter();
 	InitThingdef();
 	lastlump = 0;