Add minimized version of projection caching example

[Aaron1011]

In rust-lang/rust#89831 (comment), a build went from taking 3 minutes to 12 hours. Nothing in our current benchmark suite reflected this regressions (and the fix showed "This benchmark run did not return any relevant changes."). We should add a reduced version of that example which captures the enormous impact of projection cache 'completion' on certain workloads.

[Mark-Simulacrum]

Do we have any example code for that case yet? I definitely agree that we should add something to this effect to the benchmark suite, whether as a real-world test case or a manufactured stress test.

[Aaron1011]

I don't know if we have a concrete example. @nayato: Are you able to share the codebase that experienced the regression you described in rust-lang/rust#89831 (comment)?