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 changes adds dependencies on the legacy layouts and the clang
headers. The legacy layouts are 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. The clang headers are another
dependency of the swiftCore, which otherwise causes problems to not find
the compiler headers when building Cxx. This is necessary for builds
that use a previously built compiler to compile the stdlib.

Author: drodriguez

CMakeLists.txt

@@ -1282,7 +1282,7 @@ else()
 
   # Some tools (e.g. swift-reflection-dump) rely on a host swiftRemoteInspection,
   # so ensure we build that when building tools.
-  if(SWIFT_INCLUDE_TOOLS)
+  if(SWIFT_INCLUDE_TOOLS OR SWIFT_BUILD_STDLIB_CXX_MODULE)
     add_subdirectory(stdlib/public/SwiftShims/swift/shims)
   endif()
 

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 symlink_clang_headers
     INSTALL_IN_COMPONENT compiler
     INSTALL_WITH_SHARED)