clang-include-fixer.el: Don't prompt the user about killing clang-include-fixer processes

By default, Emacs prompts the user when killing processes on exit. This is useful for stateful processes such as interactive shells. However, clang-include-fixer processes are stateless; the only effect of killing them is to cancel a clang-include-fixer operation. Therefore prompting the user is just a nuisance.

Patch by Philipp Stephani.

Differential Revision: https://reviews.llvm.org/D25378

llvm-svn: 283863