add format target to cmake

[strega-nil-ms]

This replaces the parallelize.cpp hack, as well as allows users to run format with CMake.

Additionally, this publishes the diff, so that people without the specific version of clang-format on their machine can download the diff and apply it themselves.

[StephanTLavavej]

Looks great! I've verified the failure experiences in #2681 and #2682, and locally verified that CMake's printed messages are expected. The clang-format experience is a little different from the current one (have to click through to see the diff) but I'm ok with that behavioral change.

I went ahead and pushed a message clarification to create-prdiff.ps1 to help contributors seeing this output for the first time, and some changes to validate.cpp - minor style issues plus a rework of how any_errors are recorded to reduce verbosity and ensure that Code Format Validation will always fail if we print a failure message (this restores detection of long paths, paths with spaces, and the new bad extensions). I believe that this preserves your design intent but please meow if you disagree!

[CaseyCarter]

The clang-format experience is a little different from the current one (have to click through to see the diff) but I'm ok with that behavioral change.

I'm mildly concerned that this will mean we spend a lot more time explaining to new users how to find the formatting diff, but we can wait to address it if and when it actually becomes a problem.

[StephanTLavavej]

Closing and reopening to fix the CLA check.

[StephanTLavavej]

I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.

[StephanTLavavej]

Thanks for improving this important part of our infrastructure (parallelize was a source of headaches in the past, e.g. #1619), and congratulations on your first microsoft/STL commit!

🎉 😻 🚀