Sema: Allow overrides to be unavailable in the current Swift language mode.

Fixes a regression from https://github.com/swiftlang/swift/pull/83354.

Resolves rdar://158262522

test/Availability/availability_unavailable_overrides.swift

@@ -239,7 +239,6 @@ func testOverrideOfUnavailableDeclFromUnavailableDerivedType() {
   }
 }
 
-
 func testImplicitSuperInit() {
   // FIXME: The diagnostics for the implicit call to super.init() could be
   // relaxed since both initializers are unreachable and the developer cannot
@@ -256,3 +255,19 @@ func testImplicitSuperInit() {
     // expected-note@-2 {{call to unavailable initializer 'init()' from superclass 'Base' occurs implicitly at the end of this initializer}}
   }
 }
+
+func testUnavailableInSwiftOverrides() {
+  class Base {
+    func availableMethod() {}
+  }
+
+  class Derived1: Base {
+    @available(swift, introduced: 99)
+    override func availableMethod() {}
+  }
+
+  class Derived2: Base {
+    @available(swift, obsoleted: 1)
+    override func availableMethod() {}
+  }
+}