Fix crash with failed typecheck

The walker was crashing if something failed to typecheck since the type
wasn't available. I've added a little test to ensure that not only do we
not crash, but continue checking within the closure.

Author: etcwilde

lib/Sema/MiscDiagnostics.cpp

@@ -4629,6 +4629,9 @@ class CompletionHandlerUsageChecker final : public ASTWalker {
   bool walkToDeclPre(Decl *D) override { return !isa<PatternBindingDecl>(D); }
 
   std::pair<bool, Expr *> walkToExprPre(Expr *expr) override {
+    if (expr->getType().isNull())
+      return {false, expr}; // Something failed to typecheck, bail out
+
     if (ClosureExpr *closure = dyn_cast<ClosureExpr>(expr))
       return {closure->isBodyAsync(), closure};
 

test/attr/attr_completionhandlerasync.swift

@@ -200,4 +200,9 @@ class ClassCallingAsyncStuff {
       }
     }
   }
+
+  func instanceFunc(other: ClassCallingAsyncStuff) async {
+    // expected-error@+1{{cannot find 'c' in scope}}
+    c.compHandlerFunc() { }
+  }
 }