Diagnostic circular class inheritance.

Break cycles agressively when we find circular class inheritance. The
stronger AST invariants prevent us from having to check for loops
everywhere in the front end.


Swift SVN r7325

lib/Serialization/ModuleFile.cpp

@@ -1135,6 +1135,7 @@ Decl *ModuleFile::getDecl(DeclID DID, Optional<DeclContext *> ForcedContext,
     assert(members.hasValue() && "could not read class members");
     theClass->setMembers(members.getValue(), SourceRange());
     theClass->setCheckedInheritanceClause();
+    theClass->setCircularityCheck(CircularityCheck::Checked);
     break;
   }