change Vim key binding for include-fixer and clang-rename

Change Vim key binding for include-fixer (`,cf` -> `<leader>cf`) and
clang-rename (`,cr` -> `<leader>cr`) to use `<leader>` instead of `,` like
cool Vim people (tm) do.

Reviewers: ioeric

Differential Revision: https://reviews.llvm.org/D22854

llvm-svn: 276870