tools: add make format-cpp to run clang-format on C++ diffs

[joyeecheung]

This is a rework of #16122. I took the advice from @codebytere and make clang-format run on C++ diffs instead of the whole code base (this is also what chromium's git-cl does). This allows us to gradually format the code base while we update the C++ files in normal PRs (although it may require support from external tooling and build to make sure people run it before landing PRs).

There is currently only a shortcut in Makefile. To run it on Windows one will either need to start the command manually or refactor vcbuild.bat a bit to include/exclude the formatable C++ files in the command.

One thing to note: the clang-format npm package is maintained by the Angular team. It directly downloads executables hosted in their repository: https://github.com/angular/clang-format It seems to be reasonably safe to use. Otherwise we will have to build and host a bunch of executables for different platforms ourselves because different versions of clang-format may format the files differently so we would want to keep the version fixed.

------ patch description ----

This patch adds a make format-cpp shortcut to the Makefile
that runs clang-format on the C++ diffs, and a
make format-cpp-build to install clang-format from
npm.

To format staged changes:

$ make format-cpp

To format HEAD~1...HEAD (latest commit):

$ CLANG_FORMAT_START=`git rev-parse HEAD~1` make format-cpp

To format diff between master and current branch head (master...HEAD):

$ CLANG_FORMAT_START=master make format-cpp

Most of the .clang-format file comes from running

$ clang-format --dump-config --style=Google

with clang-format built with llvm/trunk 328768 (npm version 1.2.3)

The clang-format version is fixed because different version of
clang-format may format the files differently.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines