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 +1,19 @@
+// 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 Derived: SecureCodingObject {
+  // CHECK-NO-STRIP: define {{.*}} @"$s4Test7DerivedC5coderACSgSo7NSCoderC_tcfc"
+  // CHECK-STRIP-NOT: define {{.*}} @"$s4Test7DerivedC5coderACSgSo7NSCoderC_tcfc"
+
+  // CHECK-NO-STRIP: define {{.*}} @"$s4Test7DerivedC5coderACSgSo7NSCoderC_tcfcTo"
+  // CHECK-STRIP-NOT: define {{.*}} @"$s4Test7DerivedC5coderACSgSo7NSCoderC_tcfcTo"
+  @available(*, unavailable)
+  required init?(coder: NSCoder) {
+    fatalError()
+  }
+}

test/Inputs/clang-importer-sdk/usr/include/Foundation.h

@@ -315,6 +315,11 @@ typedef NSPoint *NSPointArray;
 - (void)setObject:(id)object forKeyedSubscript:(id)key;
 @end
 
+@interface SecureCodingObject: NSObject <NSSecureCoding>
+- (instancetype)init;
+- (nullable instancetype)initWithCoder:(NSCoder *)aDecoder;
+@end
+
 //===---
 // Enums.
 //===---