[cxx-interop] Build the C++ stdlib overlay when SWIFT_BUILD_SDK_OVERLAY=NO

The C++ stdlib overlay binaries (`libswiftCxx` and `libswiftCxxStdlib`) are now a part of the toolchain, not the OS SDKs. This makes sure that these binaries are built when building the toolchain separately from the stdlib.

rdar://105799283

Author: egorzhdan

stdlib/public/CMakeLists.txt

@@ -62,6 +62,7 @@ endif()
 
 add_subdirectory(SwiftShims/swift/shims)
 add_subdirectory(CommandLineSupport)
+add_subdirectory(Cxx)
 add_subdirectory(Threading)
 
 # This static library is shared across swiftCore and swiftRemoteInspection
@@ -159,7 +160,6 @@ endif()
 
 if(SWIFT_BUILD_SDK_OVERLAY OR SWIFT_BUILD_TEST_SUPPORT_MODULES)
   add_subdirectory(Platform)
-  add_subdirectory(Cxx)
 endif()
 
 if(SWIFT_BUILD_SDK_OVERLAY)