IRGen: Strongly link unavailable declarations with introduced: versions.

If a declaration is unavailable but also has an `introduced:` availability
version, treat that as an indication that it is considered ABI and should be
linked as if it were available.

Part of rdar://106673713

Author: tshortli

lib/AST/Decl.cpp

@@ -1126,7 +1126,12 @@ bool Decl::isAlwaysWeakImported() const {
   if (getAttrs().hasAttribute<WeakLinkedAttr>())
     return true;
 
-  if (getSemanticUnavailableAttr())
+  // Declarations that are unavailable should be weak linked since they are
+  // meant to be unreachable at runtime and their removal should not affect
+  // clients. However, make an exception for unavailable declarations with
+  // explicit introduction versions, which are considered required ABI.
+  if (getSemanticUnavailableAttr() &&
+      getAvailabilityForLinkage().isAlwaysAvailable())
     return true;
 
   if (auto *accessor = dyn_cast<AccessorDecl>(this))

test/IRGen/Inputs/weak_import_availability_helper.swift

@@ -4,6 +4,10 @@ public func conditionallyAvailableFunction() {}
 @available(macOS, unavailable)
 public func unavailableFunction() {}
 
+@available(macOS 10.50, *)
+@available(macOS, unavailable)
+public func unavailableButIntroducedFunction() {}
+
 @available(macOS 10.50, *)
 public var conditionallyAvailableGlobal: Int {
   get {return 0}

test/IRGen/weak_import_availability.swift

@@ -60,6 +60,14 @@ public func callUnavailableFunction() {
 
 // CHECK-LABEL: declare extern_weak swiftcc void @"$s31weak_import_availability_helper19unavailableFunctionyyF"()
 
+@available(macOS, unavailable)
+public func callUnavailableButIntroducedFunction() {
+  unavailableButIntroducedFunction()
+}
+
+// CHECK-OLD-LABEL: declare extern_weak swiftcc void @"$s31weak_import_availability_helper32unavailableButIntroducedFunctionyyF"()
+// CHECK-NEW-LABEL: declare swiftcc void @"$s31weak_import_availability_helper32unavailableButIntroducedFunctionyyF"()
+
 @available(macOS 10.50, *)
 public func useConditionallyAvailableGlobal() {
   _ = conditionallyAvailableGlobal