Support `./x.py run miri`

[RalfJung]

It would be great to have a way to run the in-tree Miri on a given file (similar to ./miri run in the Miri repo). That should even work with the stage 0 Miri, given that Miri's test suite also passes for stage 0.

However, I am not sure what the best x.py interface for this would look like, and how to integrate it. Cc @Mark-Simulacrum

[Mark-Simulacrum]

Can you point me to documentation or explain what exactly is being asked for here, in particular, I presume cargo miri run and cargo miri test are what we want to do, but I am not sure what those two commands do under the hood. Should I just read https://github.com/rust-lang/miri/blob/master/cargo-miri/bin.rs?

[jyn514]

./x.py run rustdoc f.rs would also be nice as an alias for x.py build src/tools/rustdoc library/std && rustdoc +stage1 f.rs.

[RalfJung]

@Mark-Simulacrum basically, it should do the equivalent of this. Up until here, it's basically the same as what happens for testing, but then the final step "Run cargo test" needs to be replaced by running the Miri driver binary instead. I'm happy to help with that part.

[camelid]

It would be nice if we had this for rustdoc, which would let people get around the issue described here. Something like ./x.py run rustdoc.