IRGen: Avoid emitting Obj-C method metadata for unavailable methods.

Resolves rdar://109911571

Author: tshortli

lib/IRGen/GenClass.cpp

@@ -1831,6 +1831,9 @@ namespace {
 
     void buildMethod(ConstantArrayBuilder &descriptors,
                      AbstractFunctionDecl *method) {
+      if (Lowering::shouldSkipLowering(method))
+        return;
+
       auto accessor = dyn_cast<AccessorDecl>(method);
       if (!accessor)
         return emitObjCMethodDescriptor(IGM, descriptors, method);

test/IRGen/unavailable_decl_optimization_complete_objc.swift

@@ -0,0 +1,17 @@
+// RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk) -parse-as-library -module-name Test -validate-tbd-against-ir=missing %s -emit-ir | %FileCheck %s --check-prefixes=CHECK,CHECK-NO-STRIP
+
+// RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk) -parse-as-library -module-name Test -validate-tbd-against-ir=missing -unavailable-decl-optimization=complete %s -emit-ir | %FileCheck %s --check-prefixes=CHECK,CHECK-STRIP
+
+// REQUIRES: objc_interop
+
+import Foundation
+
+public class Class: NSObject {
+  // CHECK-NO-STRIP: define {{.*}} @"$s4Test5ClassC3fooyyF"
+  // CHECK-STRIP-NOT: define {{.*}} @"$s4Test5ClassC3fooyyF"
+
+  // CHECK-NO-STRIP: define {{.*}} @"$s4Test5ClassC3fooyyFTo"
+  // CHECK-STRIP-NOT: define {{.*}} @"$s4Test5ClassC3fooyyFTo"
+  @available(*, unavailable)
+  @objc public func foo() {}
+}