Add .swiftinterface tests to check backward compatibility.

(cherry picked from commit 49a8770)

Author: atrick

test/SILOptimizer/lifetime_dependence/Inputs/lifetime_depend_diagnose.swiftinterface

@@ -0,0 +1,27 @@
+// swift-interface-format-version: 1.0
+// swift-module-flags: -module-name lifetime_depend_diagnose -enable-experimental-feature LifetimeDependence -swift-version 5 -enable-library-evolution
+import Swift
+import _Concurrency
+import _StringProcessing
+import _SwiftConcurrencyShims
+
+#if $LifetimeDependence
+public struct NE : ~Swift.Escapable {
+  @usableFromInline
+  internal let _pointer: Swift.UnsafeRawPointer?
+
+  @lifetime(borrow pointer)
+  public init(pointer: Swift.UnsafeRawPointer?) {
+    self._pointer = pointer
+  }
+}
+
+extension NE {
+  // This is illegal at the source level because NE.init is implicitly @lifetime(borrow),
+  // so we can't return it as dependent on @lifetime(copy self).
+  @lifetime(copy self)
+  @unsafe @_alwaysEmitIntoClient public func forward() -> NE {
+    return NE(pointer: _pointer)
+  }
+}
+#endif

test/SILOptimizer/lifetime_dependence/diagnose_interface.swift

@@ -0,0 +1,13 @@
+// RUN: %target-swift-frontend %s -emit-sil \
+// RUN:   -o /dev/null \
+// RUN:   -I %S/Inputs \
+// RUN:   -verify \
+// RUN:   -enable-experimental-feature LifetimeDependence
+
+// REQUIRES: swift_in_compiler
+// REQUIRES: swift_feature_LifetimeDependence
+
+// Test that lifetime dependence diagnostics continues to older (early
+// 2025) .swiftinterface files. Source-level diagnostics are stricter.
+
+import lifetime_depend_diagnose

test/Sema/Inputs/lifetime_depend_infer.swiftinterface

@@ -1,7 +1,5 @@
 // swift-interface-format-version: 1.0
-// swift-compiler-version: Swift version 6.2-dev effective-5.10 (LLVM 09f3cd831902283, Swift 889522485775a5d)
 // swift-module-flags: -module-name lifetime_depend_infer -enable-experimental-feature LifetimeDependence -swift-version 5 -enable-library-evolution
-// swift-module-flags-ignorable:  -formal-cxx-interoperability-mode=off -interface-compiler-version 6.2
 import Swift
 import _Concurrency
 import _StringProcessing

test/Sema/lifetime_depend_infer_interface.swift

@@ -7,6 +7,6 @@
 
 // Test that type checking continues to handle inference of lifetime
 // dependencies that may be required in older (early
-// 2025) .swiftinterface files. Source-level type checking is more strict.
+// 2025) .swiftinterface files. Source-level type checking is stricter.
 
 import lifetime_depend_infer