[clang-rename] Overhaul clang-rename.el.

* Use lexical binding, as recommended for new libraries.
* Fix customization variable (set correct group and type).
* Create a new customization group for the library.
* Autoload the main clang-rename command so that users don't have to explicitly load the library.
* Correctly translate between file and buffer positions using bufferpos-to-filepos (if available) or a fallback.
* Don't invoke the shell, it's not necessary and adds complexity.
* Save clang-rename output in a buffer and display that on failure.
* Save all buffers before calling clang-rename. This is required anyway and prevents data loss when the buffer is later reverted.
* In revert-buffer, use keywords instead of t for Boolean arguments to improve readability.
* Don't reset buffer modes when reverting.
* Emacs treats the character after a symbol as part of the symbol, while clang-rename doesn't; resolve this inconsistency.
* Formatting.

Patch by Philipp Stephani!

Reviewers: omtcyfz

Differential Revision: https://reviews.llvm.org/D25156

llvm-svn: 283067