Fix warnings in clang-completion-mode.el.
Fix warnings in clang-completion-mode.el.

- Use defvar to declare variables
- Don't use delete-backward-char, which is for interactive use only

Patch by Philipp Stephani

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@282573 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,apple/swift-clang