Include AddLLVM needed for tests in the right context

AddLLVM is needed for several functions that are used in tests and
as such needs to be included from the right context which previously
wasn't the case.

Differential Revision: https://reviews.llvm.org/D40280

llvm-svn: 319515
diff --git a/libcxx/CMakeLists.txt b/libcxx/CMakeLists.txt
index f0c4d1e..29eef8f 100644
--- a/libcxx/CMakeLists.txt
+++ b/libcxx/CMakeLists.txt
@@ -684,6 +684,7 @@
 endif()
 
 if (LIBCXX_STANDALONE_BUILD AND EXISTS "${LLVM_MAIN_SRC_DIR}/utils/llvm-lit")
+  include(AddLLVM) # for get_llvm_lit_path
   # Make sure the llvm-lit script is generated into the bin directory, and do
   # it after adding all tests, since the generated script will only work
   # correctly discovered tests against test locations from the source tree that