[cxx-interop] Do not emit C++ interop flag in textual interfaces

This makes sure that the compiler does not emit `-enable-experimental-cxx-interop`/`-cxx-interoperability-mode` flags in `.swiftinterface` files. Those flags were breaking explicit module builds. The module can still be rebuilt from its textual interface if C++ interop was enabled in the current compilation.

rdar://140203932

Author: egorzhdan

include/swift/Option/Options.td

@@ -794,12 +794,12 @@ def enable_experimental_concise_pound_file : Flag<["-"],
 
 def enable_experimental_cxx_interop :
   Flag<["-"], "enable-experimental-cxx-interop">,
-  Flags<[NoDriverOption, FrontendOption, HelpHidden, ModuleInterfaceOption]>,
+  Flags<[NoDriverOption, FrontendOption, HelpHidden]>,
   HelpText<"Enable experimental C++ interop code generation and config directives">;
 
 def cxx_interoperability_mode :
   Joined<["-"], "cxx-interoperability-mode=">,
-  Flags<[FrontendOption, ModuleInterfaceOption, SwiftSymbolGraphExtractOption,
+  Flags<[FrontendOption, SwiftSymbolGraphExtractOption,
          SwiftSynthesizeInterfaceOption]>,
   HelpText<"Enables C++ interoperability; pass 'default' to enable or 'off' to disable">;
 

lib/Frontend/ModuleInterfaceLoader.cpp

@@ -2014,12 +2014,7 @@ InterfaceSubContextDelegateImpl::InterfaceSubContextDelegateImpl(
     GenericArgs.push_back(blocklist);
   }
 
-  // For now, we only inherit the C++ interoperability mode in
-  // Explicit Module Builds.
-  if (langOpts.EnableCXXInterop &&
-      (frontendOpts.DisableImplicitModules ||
-       LoaderOpts.requestedAction ==
-           FrontendOptions::ActionType::ScanDependencies)) {
+  if (langOpts.EnableCXXInterop) {
     // Modelled after a reverse of validateCxxInteropCompatibilityMode
     genericSubInvocation.getLangOptions().EnableCXXInterop = true;
     genericSubInvocation.getLangOptions().cxxInteropCompatVersion =

test/Interop/Cxx/modules/emit-module-interface.swift

@@ -3,15 +3,18 @@
 // Check if fragile Swift interface with struct
 // extensions can be reparsed:
 // RUN: %target-swift-frontend -swift-version 5 -typecheck -emit-module-interface-path %t/UsesCxxStruct.swiftinterface %s -I %S/Inputs -swift-version 5 -enable-experimental-cxx-interop %S/Inputs/namespace-extension-lib.swift
-// RUN: %target-swift-frontend -swift-version 5 -typecheck-module-from-interface  %t/UsesCxxStruct.swiftinterface -I %S/Inputs
+// RUN: %target-swift-frontend -swift-version 5 -typecheck-module-from-interface  %t/UsesCxxStruct.swiftinterface -I %S/Inputs -enable-experimental-cxx-interop
 // RUN: %FileCheck --input-file=%t/UsesCxxStruct.swiftinterface %s
-// CHECK: -enable-experimental-cxx-interop
+
+// The textual module interface should not contain the C++ interop flag.
+// CHECK-NOT: -enable-experimental-cxx-interop
+// CHECK-NOT: -cxx-interoperability-mode
 
 
 // Check if resilient Swift interface with builtin
 // type extensions can be reparsed:
 // RUN: %target-swift-emit-module-interface(%t/ResilientStruct.swiftinterface) %s -I %S/Inputs -enable-library-evolution -swift-version 5 -enable-experimental-cxx-interop %S/Inputs/namespace-extension-lib.swift -DRESILIENT
-// RUN: %target-swift-typecheck-module-from-interface(%t/ResilientStruct.swiftinterface) -I %S/Inputs -DRESILIENT
+// RUN: %target-swift-typecheck-module-from-interface(%t/ResilientStruct.swiftinterface) -I %S/Inputs -DRESILIENT -enable-experimental-cxx-interop
 // RUN: %FileCheck --input-file=%t/ResilientStruct.swiftinterface %s
 
 import Namespaces