build: Fixes for building Cxx module with/without stdlib.

With the changes introduced in #65055, the Cxx module will build even
when not building the stdlib as long as
`SWIFT_BUILD_STDLIB_EXTRA_TOOLCHAIN_CONTENT` was set. With the further changes
in #65122 the previous flag is not necessary, and only
`SWIFT_BUILD_STDLIB_CXX_MODULE` (which defaults to `TRUE`) is necessary.

However, `stdlib/public/Cxx` did rely on `StdlibOptions.cmake` to be
include before it, or the functions that build the target libraries will
find a half configured state. `StdlibOptions.cmake` are included in
`stdlib/toolchain`, in `stdlib/` and `StandaloneOverlay.cmake`. Before
 #65122 it was working just because `stdlib/toolchain` was added just
before.

The second change adds a dependency on the legacy layouts, which is
a dependency only added to the libraries of the stdlib core
(IS_STDLIB_CORE flag). Since enabling that flag also enables a bunch
other stuff, and I am not sure that Cxx should be classified as "core"
anyway, I preferred to add the dependency. This is necessary for builds
like the iPhoneOS and iPhoneSimulator, and does not seem to affect
macOS.

Author: drodriguez

stdlib/public/Cxx/CMakeLists.txt

@@ -1,3 +1,6 @@
+list(APPEND CMAKE_MODULE_PATH ${CMAKE_CURRENT_LIST_DIR}/../../cmake/modules)
+include(StdlibOptions)
+
 set(SWIFT_CXX_LIBRARY_KIND STATIC)
 if("${SWIFT_HOST_VARIANT_SDK}" STREQUAL "WINDOWS")
   set(SWIFT_CXX_LIBRARY_KIND SHARED)
@@ -20,6 +23,7 @@ add_swift_target_library(swiftCxx ${SWIFT_CXX_LIBRARY_KIND} NO_LINK_NAME IS_STDL
     -Xcc -nostdinc++
 
     LINK_FLAGS "${SWIFT_RUNTIME_SWIFT_LINK_FLAGS}"
+    DEPENDS copy-legacy-layouts
     INSTALL_IN_COMPONENT compiler
     INSTALL_WITH_SHARED)