Sema: Relax feature check for non-escapable types in swiftinterfaces.

Checking for the presence of a feature flag in order to allow syntax tends to
result in condfails when applied to `.swiftinterfaces`, since older compilers
that have this restriction may be used to compile newer interfaces where the
restriction has been lifted. Remove this restriction for non-escapable types
when typechecking an interface.

Resolves rdar://128577611

Author: tshortli

lib/Sema/TypeCheckType.cpp

@@ -2071,6 +2071,11 @@ namespace {
       return (SF && SF->Kind == SourceFileKind::SIL);
     }
 
+    bool isInterfaceFile() const {
+      auto SF = getDeclContext()->getParentSourceFile();
+      return (SF && SF->Kind == SourceFileKind::Interface);
+    }
+
     /// Short-hand to query the current stage of type resolution.
     bool inStage(TypeResolutionStage stage) const {
       return resolution.getStage() == stage;
@@ -4782,6 +4787,7 @@ TypeResolver::resolveDeclRefTypeRepr(DeclRefTypeRepr *repr,
       if (auto known = proto->getKnownProtocol()) {
         if (*known == KnownProtocolKind::Escapable
             && !isSILSourceFile()
+            && !isInterfaceFile()
             && !ctx.LangOpts.hasFeature(Feature::NonescapableTypes)) {
           diagnoseInvalid(repr, repr->getLoc(),
                           diag::escapable_requires_feature_flag);

test/ModuleInterface/escapable.swiftinterface

@@ -0,0 +1,14 @@
+// swift-interface-format-version: 1.0
+// swift-compiler-version: Swift version 6.0
+// swift-module-flags: -swift-version 5 -enable-library-evolution -module-name Library
+
+// RUN: %target-swift-typecheck-module-from-interface(%s) -module-name Library
+
+import Swift
+import _Concurrency
+import _StringProcessing
+import _SwiftConcurrencyShims
+
+// Allow Escapable to appear without -enable-experimental-feature NonescapableTypes
+public struct S : Swift.Escapable {
+}

validation-test/ParseableInterface/rdar128577611.swift

@@ -0,0 +1,10 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-swift-emit-module-interface(%t/Library.swiftinterface) %s -module-name Library
+// RUN: %target-swift-typecheck-module-from-interface(%t/Library.swiftinterface) -module-name Library
+// RUN: %FileCheck %s < %t/Library.swiftinterface
+
+struct InternalStruct {}
+extension [Int: InternalStruct]: Sendable {}
+
+// CHECK: @available(*, unavailable)
+// CHECK: extension Swift.Dictionary : Swift.Copyable, Swift.Escapable, Swift.Sendable where Key : _ConstraintThatIsNotPartOfTheAPIOfThisLibrary {}