[libc++] Fix installation of cxx_experimental

cxx_experimental was never installed because ${experimental_lib} always
expands to an empty target. This seems to have been broken by 97d6fcce4ec6.
diff --git a/libcxx/src/CMakeLists.txt b/libcxx/src/CMakeLists.txt
index 2a9d054..63ca34d 100644
--- a/libcxx/src/CMakeLists.txt
+++ b/libcxx/src/CMakeLists.txt
@@ -349,7 +349,7 @@
   endif()
 
   if(LIBCXX_INSTALL_EXPERIMENTAL_LIBRARY)
-    install(TARGETS ${LIBCXX_INSTALL_TARGETS} ${experimental_lib}
+    install(TARGETS cxx_experimental
       LIBRARY DESTINATION ${LIBCXX_INSTALL_PREFIX}${LIBCXX_INSTALL_LIBRARY_DIR} COMPONENT cxx
       ARCHIVE DESTINATION ${LIBCXX_INSTALL_PREFIX}${LIBCXX_INSTALL_LIBRARY_DIR} COMPONENT cxx
       RUNTIME DESTINATION ${LIBCXX_INSTALL_PREFIX}bin COMPONENT cxx)