[cxx-interop] Allow disabling Cxx build when building stdlib

This makes it possible to pass `SWIFT_BUILD_STDLIB_CXX_MODULE=FALSE` to CMake to disable Cxx and CxxStdlib modules when building the Swift stdlib or the SDK overlay.

rdar://108206321

Author: egorzhdan

CMakeLists.txt

@@ -244,7 +244,7 @@ option(SWIFT_BUILD_STDLIB_EXTRA_TOOLCHAIN_CONTENT
     TRUE)
 
 option(SWIFT_BUILD_STDLIB_CXX_MODULE
-  "If not building stdlib, controls whether to build the Cxx module"
+  "Build the Cxx module and the CxxStdlib overlay module"
   TRUE)
 
 # In many cases, the CMake build system needs to determine whether to include

stdlib/public/CMakeLists.txt

@@ -65,7 +65,9 @@ endif()
 
 add_subdirectory(SwiftShims/swift/shims)
 add_subdirectory(CommandLineSupport)
-add_subdirectory(Cxx)
+if(SWIFT_BUILD_STDLIB_CXX_MODULE)
+  add_subdirectory(Cxx)
+endif()
 add_subdirectory(Threading)
 
 # This static library is shared across swiftCore and swiftRemoteInspection