AST: Make ConstructorDecl::getInitializerInterfaceType() more forgiving of invalid code

Author: slavapestov

lib/AST/Decl.cpp

@@ -6856,7 +6856,13 @@ Type ConstructorDecl::getInitializerInterfaceType() {
     return InitializerInterfaceType;
 
   // Lazily calculate initializer type.
-  auto funcTy = getInterfaceType()->castTo<AnyFunctionType>()->getResult();
+  auto allocatorTy = getInterfaceType();
+  if (!allocatorTy->is<AnyFunctionType>()) {
+    InitializerInterfaceType = ErrorType::get(getASTContext());
+    return InitializerInterfaceType;
+  }
+
+  auto funcTy = allocatorTy->castTo<AnyFunctionType>()->getResult();
   assert(funcTy->is<FunctionType>());
 
   // Constructors have an initializer type that takes an instance