[Effects checking] Handle optional function types for dynamic @objc calls

We've been missing this case forever, but it hadn't come up until
we performed "unsafe" checking on calls.

(cherry picked from commit 60f7d20)

Author: DougGregor

lib/Sema/TypeCheckEffects.cpp

@@ -385,15 +385,18 @@ class AbstractFunction {
 
   Type getType() const {
     switch (getKind()) {
-    case Kind::Opaque: return getOpaqueFunction()->getType();
+    case Kind::Opaque:
+      return getOpaqueFunction()->getType()->lookThroughSingleOptionalType();
     case Kind::Function: {
       auto *AFD = getFunction();
       if (AFD->hasImplicitSelfDecl() && AppliedSelf)
         return AFD->getMethodInterfaceType();
       return AFD->getInterfaceType();
     }
     case Kind::Closure: return getClosure()->getType();
-    case Kind::Parameter: return getParameter()->getInterfaceType();
+    case Kind::Parameter:
+      return getParameter()->getInterfaceType()
+          ->lookThroughSingleOptionalType();
     }
     llvm_unreachable("bad kind");
   }
@@ -1831,6 +1834,11 @@ class ApplyClassifier {
           return;
         }
 
+        // Can end up with an optional type when dynamically dispatching to
+        // @objc.
+        if (auto optFnType = fnInterfaceType->getOptionalObjectType())
+          fnInterfaceType = optFnType;
+
         // Use the most significant result from the arguments.
         FunctionType *fnSubstType = nullptr;
         if (auto *fnGenericType = fnInterfaceType->getAs<GenericFunctionType>())