Minor cleanups in clang-format.el.

- Enable lexical binding
- Formatting
- Enable file name completion for the clang-format-executable variable
- Add a missing docstring
- When available, use bufferpos-to-filepos and filepos-to-bufferpos. These functions given more precise mapping than byte-to-position and position-bytes.
- Rename arguments of clang-format-region to match the docstring
- Instead of binding local variables to nil and then assigning them, bind them directly to their values
- Make use of the fact that insert-file-contents returns the number of characters it inserted
- Use cl-destructuring-bind to make the code a bit shorter
- Use standard iteration (dolist) instead of mapc with a lambda, which is more common and shorter
- Remove a message that was most likely only present for debugging purposes

Patch by Philipp Stephani.

llvm-svn: 283206