[cmake] Work around a bug in the llvm config.

In short we use variables which require including `GNUInstallDirs` but we are
expecting somebody else to include it for us.

See llvm/llvm-project#83802

Author: vgvassilev

CMakeLists.txt

@@ -1,5 +1,7 @@
 cmake_minimum_required(VERSION 3.13)
 
+include(GNUInstallDirs)
+
 set(CMAKE_MODULE_PATH
   ${CMAKE_MODULE_PATH}
   "${CMAKE_CURRENT_SOURCE_DIR}/cmake"