This fixes a compiler bug that got exposed by f11abac652.
If a C++ type is declared in a nested Clang submodule, Swift was emitting errors that look like:
```
Type alias 'string' is not available due to missing import of defining module 'fwd’
```
rdar://146899125
This makes sure that when Swift is generating a `.swiftinterface` file for a Swift module with a dependency on C++ module, `-enable-experimental-cxx-interop` is emitted under `// swift-module-flags:`.
The module interface might refer to C++ symbols which are not available in Swift without C++ interop enabled. This caused a build error for Swift LLVM bindings during `verify-module-interface` stage: Swift tried to import the module interface and couldn't find C++ stdlib headers because C++ interop was disabled.
