bootstrap: add ./x run compiler-coverage for measuring rustc's own test suite coverage

[akintewe]

This is a draft PR for a ./x run compiler-coverage step that measures how much of rustc's own source is exercised by its test suite (ui, incremental, run-make, and so on).

It builds an instrumented rustc, runs the test suites against it, merges the resulting profile data, and produces an HTML report broken down by crate, file, and function, showing which parts of the compiler are and aren't covered.

LLM disclosure

I used an LLM which was Claude, via Claude Code throughout this project as a substantial collaborator, not just for cleanup or review. It wrote most of the CompilerCoverage bootstrap step, the profraw merging and callback logic in test.rs, and most of the coverage transform logic like monomorphization merging and closure inlining. It also found and fixed several real bugs like a profraw race condition, a stage instrumentation scoping bug, a stale profdata bug, and handled rebasing this branch onto current main and resolving conflicts.

I wrote the HTML report generation, its CSS, render.rs, and main.rs myself, along with part of transform.rs. I understand the code well enough to walk through and defend any part of it, and this disclosure is meant to reflect accurately where the LLM's contribution was substantial versus where the implementation was mine.

[rust-bors]

☔ The latest upstream changes (presumably #161196) made this pull request unmergeable. Please resolve the merge conflicts by rebasing.