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

.arclint

@@ -0,0 +1,12 @@
+{
+  "linters": {
+    "clang-format": {
+      "type": "script-and-regex",
+      "script-and-regex.script": "utils/arcanist/clang-format.sh",
+      "script-and-regex.regex": "/^(?P<severity>[[:alpha:]]+)\n(?P<message>[^\n]+)\n(====|(?P<line>\\d),(?P<char>\\d)\n(?P<original>.*)>>>>\n(?P<replacement>.*)<<<<\n)$/s",
+      "include": [
+        "(\\.(cc|cpp|h)$)"
+      ]
+    }
+  }
+}

utils/arcanist/clang-format.sh

@@ -0,0 +1,59 @@
+#!/bin/bash
+
+set -euo pipefail
+
+# "script-and-regex.regex": "/^(?P<severity>.*?)\n(?P<message>.*?)\n(?P<line>\\d),(?P<char>\\d)(\n(?P<original>.*?)>>>>\n(?P<replacement>.*?)<<<<?)$/s",
+
+# Arcanist linter that invokes clang-format.
+# stdout from this script is parsed into a regex and used by Arcanist.
+# https://secure.phabricator.com/book/phabricator/article/arcanist_lint_script_and_regex/
+
+# To skip running all linters when creating/updating a diff, use `arc diff --nolint`.
+
+if ! hash clang-format-diff >/dev/null; then
+  # advice severity level is completely non-disruptive.
+  # switch to warning or error if you want to prompt the user.
+  echo "advice"
+  echo "clang-format-diff not found in user's PATH; not linting file."
+  echo "===="
+  exit 0
+fi
+
+src_file="${1}"
+original_file="$(mktemp)"
+formatted_file="$(mktemp)"
+readonly src_file
+readonly original_file
+readonly formatted_file
+cp -p "${src_file}" "${original_file}"
+cp -p "${src_file}" "${formatted_file}"
+
+cleanup() {
+  rc=$?
+  rm "${formatted_file}" "${original_file}"
+  exit ${rc}
+}
+trap 'cleanup' INT HUP QUIT TERM EXIT
+
+# Arcanist can filter out lint messages for unchanged lines, but for that, we
+# need to generate line by line lint messages. Instead, we generate one lint
+# message on line 1, char 1 with file content edited using clang-format-diff.
+if git rev-parse --git-dir >/dev/null; then
+  arc_base_commit=$(arc which --show-base)
+  # An alternative is to use git-clang-format.
+  git diff -U0 --no-color "${arc_base_commit}"| clang-format-diff -style LLVM -i -p1
+else
+  svn diff --diff-cmd=diff -x -U0 "${src_file}" | clang-format-diff -style LLVM -i
+fi
+
+cp -p "${src_file}" "${formatted_file}"
+cp -p "${original_file}" "${src_file}"
+if ! diff -q "${src_file}" "${formatted_file}" > /dev/null ; then
+  echo "autofix"
+  echo "clang-format suggested style edits found:"
+  echo "1,1"  # line,char of start of replacement.
+  cat "${src_file}"
+  echo ">>>>"
+  cat "${formatted_file}"
+  echo "<<<<"
+fi