Index: CMakeLists.txt
===================================================================
--- CMakeLists.txt
+++ CMakeLists.txt
@@ -117,7 +117,7 @@
   endif()
   string(TOUPPER "${proj}" upper_proj)
   STRING(REGEX REPLACE "-" "_" upper_proj ${upper_proj})
-  set(LLVM_EXTERNAL_${upper_proj}_SOURCE_DIR   "${CMAKE_CURRENT_SOURCE_DIR}/../${proj}")
+  set(LLVM_EXTERNAL_${upper_proj}_SOURCE_DIR   "${CMAKE_CURRENT_SOURCE_DIR}/../${proj}" CACHE STRING "")
   # There is a widely spread opinion that clang-tools-extra should be merged
   # into clang. The following simulates it by always enabling clang-tools-extra
   # when enabling clang.