TableGen: Explicitly forbid some nestings of class, multiclass, and foreach

[nhaehnle]

Summary

These previously all failed one way or another, but now we produce a more
helpful error message.

Change-Id: I8ffd2e87c8e35a5134c3be289e0a1fecaa2bb8ca

lib/TableGen/TGParser.cpp

bool TGParser::ParseObject(MultiClass *MC) {
   case tgtok::Let:   return ParseTopLevelLet(MC);
   case tgtok::Def:   return ParseDef(MC);
   case tgtok::Foreach:   return ParseForeach(MC);
   case tgtok::Defm:  return ParseDefm(MC);
   case tgtok::Defset:
     if (MC)
       return TokError("defset is not allowed inside multiclass");
     return ParseDefset();
-  case tgtok::Class: return ParseClass();
-  case tgtok::MultiClass: return ParseMultiClass();
+  case tgtok::Class:
+    if (MC)
+      return TokError("class is not allowed inside multiclass");
+    if (!Loops.empty())
+      return TokError("class is not allowed inside foreach loop");
+    return ParseClass();
+  case tgtok::MultiClass:
+    if (!Loops.empty())
+      return TokError("multiclass is not allowed inside foreach loop");
+    return ParseMultiClass();
   }
 }
 
 /// ParseObjectList
 ///   ObjectList :== Object*
 bool TGParser::ParseObjectList(MultiClass *MC) {
   while (isObjectStart(Lex.getCode())) {
     if (ParseObject(MC))