Improving build times

Use maintainers/buildtime_report.sh build/ in a clang stdenv when building with Meson. This will simply tell you where all our build time went.

Builds with clang and meson are profiled by default, because we want to encourage improving the build time.

If you want to stare down a specific file harder, look in build/ for the relevant json file and import it to https://ui.perfetto.dev.