as part of the Chromium checkout.

This follows the approach used by gn.

Changes include:
 - in-the-PATH clang-format trampoline scripts
 - clang_format.py, which finds clang-format binaries inside of Chrome
 - Hook 'git cl format' to the new binaries and scripts
 - Rearrange some code, for reuse between clang_format.py and gn.py

BUG=240309
TEST=presubmits (one failure on mac, but it fails on a clean checkout too)

Review URL: https://codereview.chromium.org/134313007

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@245074 0039d316-1c4b-4281-b951-d872f2087c98