Use UTF-8 for all communication with clang-format

Summary: Instead of picking the buffer file coding system, always use utf-8-unix for communicating with clang-format.  This is fine because clang-format never actually reads the file to be formatted, only standard input.  This is a bit simpler (process coding system is now a constant) and potentially faster, as utf-8-unix is Emacs's internal coding system.  Also add an end-to-end test that actually invokes clang-format.

Reviewers: klimek

Reviewed By: klimek

Differential Revision: https://reviews.llvm.org/D28904

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@292593 91177308-0d34-0410-b5e6-96231b3b80d8