Add -Wdangling-else.

This works like described in  http://drdobbs.com/blogs/cpp/231602010
Fixes http://llvm.org/PR11609



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@147202 91177308-0d34-0410-b5e6-96231b3b80d8
5 files changed