Index: cmake/modules/AddLLVM.cmake
===================================================================
--- cmake/modules/AddLLVM.cmake
+++ cmake/modules/AddLLVM.cmake
@@ -193,7 +193,17 @@
 # Set each output directory according to ${CMAKE_CONFIGURATION_TYPES}.
 # Note: Don't set variables CMAKE_*_OUTPUT_DIRECTORY any more,
 # or a certain builder, for eaxample, msbuild.exe, would be confused.
-function(set_output_directory target bindir libdir)
+function(set_output_directory target)
+  # This function takes bindir and libdir arguments but allows them to be unset,
+  # in which case the default output directory is used (this is useful when
+  # using AddLLVM.cmake from an installed LLVM toolchain as they will end up
+  # being unset in that case). This is done by leaving those arguments off the
+  # argument list and instead getting them from ARGV.
+  if(${ARGC} LESS 3)
+    return()
+  endif()
+  set(bindir ${ARGV1})
+  set(libdir ${ARGV2})
   # Do nothing if *_OUTPUT_INTDIR is empty.
   if("${bindir}" STREQUAL "")
     return()