parallel with the rest of the tools directory as it depends on Clang.

This patch was first applied in r125956 and subsequently reverted in
r125964 as it broke in-tree builds.  Makefile.rules was fixed up in 
r126070 to handle missing optional directories for the in-tree case,
so it should be safe now to bring this patch back in.
 

llvm-svn: 126071