Merge pull request #64250 from apple/egorzhdan/enable-cxxstdlib

[cxx-interop] Build `CxxStdlib` module when `SWIFT_BUILD_SDK_OVERLAY=NO`

Author: egorzhdan

stdlib/public/Cxx/CMakeLists.txt

@@ -23,7 +23,5 @@ add_swift_target_library(swiftCxx ${SWIFT_CXX_LIBRARY_KIND} NO_LINK_NAME IS_STDL
     INSTALL_IN_COMPONENT compiler
     INSTALL_WITH_SHARED)
 
-if(SWIFT_BUILD_SDK_OVERLAY OR SWIFT_BUILD_TEST_SUPPORT_MODULES)
-  add_subdirectory(std)
-endif()
+add_subdirectory(std)
 add_subdirectory(cxxshim)