[CodeCompletion] Don't complete unavailable methods in override

We generally don't want to override unavailable methods.  We already
filter out unavailable decls when completing values, but that's in a
separate visible decl consumer.

rdar://problem/25058233

Author: benlangmuir

lib/IDE/CodeCompletion.cpp

@@ -3795,6 +3795,9 @@ class CompletionOverrideLookup : public swift::VisibleDeclConsumer {
     if (Reason == DeclVisibilityKind::MemberOfCurrentNominal)
       return;
 
+    if (AvailableAttr::isUnavailable(D))
+      return;
+
     if (D->getAttrs().hasAttribute<FinalAttr>())
       return;
 

test/IDE/complete_not_recommended.swift

@@ -4,6 +4,8 @@
 // CHECK1: Decl[Constructor]/CurrNominal:      init()[#A#]; name=init(){{$}}
 // CHECK1: End completions
 
+// RUN: %target-swift-ide-test -code-completion -source-filename %s -code-completion-token=OVERRIDE_1 | FileCheck %s -check-prefix=OVERRIDE_1
+
 class A {
   func foo() {}
   @available(*, unavailable) func unavail() {}
@@ -12,3 +14,12 @@ class A {
 func glob() {
   A.#^MEMBER^#
 }
+
+class B : A {
+  override func #^OVERRIDE_1^#
+// OVERRIDE_1: Begin completions
+// OVERRIDE_1-NOT: Decl[InstanceMethod]
+// OVERRIDE_1: Decl[InstanceMethod]/Super:         foo() {|};
+// OVERRIDE_1-NOT: Decl[InstanceMethod]
+// OVERRIDE_1: End completions
+}