[cxx-interop] Add missing dependency to CxxStdlib

The CxxStdlib module now relies on the libcxxshim modulemap, so we
need to make sure that the modulemap is copied into the correct location before CxxStdlib is built.

rdar://108007693
(cherry picked from commit 0303132)

Author: egorzhdan

stdlib/public/Cxx/std/CMakeLists.txt

@@ -144,4 +144,4 @@ add_swift_target_library(swiftCxxStdlib STATIC NO_LINK_NAME IS_STDLIB
     TARGET_SDKS ALL_APPLE_PLATFORMS LINUX
     INSTALL_IN_COMPONENT compiler
     INSTALL_WITH_SHARED
-    DEPENDS libstdcxx-modulemap)
+    DEPENDS libstdcxx-modulemap libcxxshim_modulemap)