Setup clang-format as an Arcanist linter

Setup clang-format as an Arcanist linter

Summary:
This uses clang-format-diff as a linter for Arcanist.

`arc lint` flow, also run as part of `arc diff` unless skipped with
`--nolint`, will now run the linter shell script on the changed files,
and prompt the user to accept the suggested changes.

Message when clang-format-diff is not installed:
{F6654094}

Example of the noise during code review when clang-format-diff is not installed:
https://reviews.llvm.org/differential/changeset/?ref=1115809

Prompt when clang-format-diff is installed and suggests edits:
{F6650223}

Reviewers: probinson, scott.linder

Reviewed By: scott.linder

Subscribers: scott.linder, MyDeveloperDay, JonasToth, danilaml, JDevlieghere, dberris, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D49116