[AST] Return null type in AbstractClosureExpr::getResultType if getType is null

If `getType` returns a `null` type, also return a `null` type in `AbstractClosureExpr::getResultType` instead of crashing.

Fixes rdar://77565983

Author: ahoppen

lib/AST/Expr.cpp

@@ -1931,10 +1931,11 @@ void AbstractClosureExpr::setParameterList(ParameterList *P) {
 Type AbstractClosureExpr::getResultType(
     llvm::function_ref<Type(Expr *)> getType) const {
   auto *E = const_cast<AbstractClosureExpr *>(this);
-  if (getType(E)->hasError())
-    return getType(E);
+  Type T = getType(E);
+  if (!T || T->hasError())
+    return T;
 
-  return getType(E)->castTo<FunctionType>()->getResult();
+  return T->castTo<FunctionType>()->getResult();
 }
 
 bool AbstractClosureExpr::isBodyThrowing() const {