build: Further fixes for Cxx w/o stdlib. (swiftlang#65217)

In swiftlang#65172 I tried to fix a problem when the Cxx module is enabled, but
we are not building the stdlib. The fix work for `swiftCxx`, but failed
to to realize that `swiftCxxStdlib` overlay is also built, which needs
parts of the stdlib.

The original swiftlang#65055 only built the Cxx module when
`EXTRA_TOOLCHAIN_CONTENT` was set, but swiftlang#65122 changed so it was two
independent checks. A more faithful fix will had been to nest the `if`
as I am trying to do with this PR. Parts of swiftlang#65172 are still necessary
to build in every case, though: the `swiftCxx` target is dependent on
the Clang headers and the legacy layouts. Those two pieces are normally
in place because of other targets in upstream builds, but fail to
materialize when the stdlib is trying to be built with the host
compiler.

This one and swiftlang#65172 will need to be cherry-picked to 5.9 because swiftlang#65055
and swiftlang#65122 where cherry-picked, so 5.9 is having the same problems as
`main`.

Author: drodriguez

CMakeLists.txt

@@ -1269,10 +1269,10 @@ else()
 
   if(SWIFT_BUILD_STDLIB_EXTRA_TOOLCHAIN_CONTENT)
     add_subdirectory(stdlib/toolchain)
-  endif()
 
-  if(SWIFT_BUILD_STDLIB_CXX_MODULE)
-    add_subdirectory(stdlib/public/Cxx)
+    if(SWIFT_BUILD_STDLIB_CXX_MODULE)
+      add_subdirectory(stdlib/public/Cxx)
+    endif()
   endif()
 
   if (BUILD_SWIFT_CONCURRENCY_BACK_DEPLOYMENT_LIBRARIES)