[docs] Fix Sphinx toctree warning.

Last I heard LSan is getting close to prime-time, but for now keep
LeakSanitizer.rst hidden (especially since it contains so little
content).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@190337 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed