x.py fix

[zackmdavis]

#53816 (migrating part of the codebase to use '_ lifetimes in type names) ended up involving quite a lot of manual text-editing labor because, unfortunately, rustfix doesn't work with x.py. (You might think that you could run cargo fix on an individual crate, but after supplying missing feature flags and environment variables (RUSTC_ERROR_METADATA_DST, CFG_COMPILER_HOST_TRIPLE), you end up triggering an assertion that I don't know how to hack around.) It would be nice if there was an x.py fix that worked (even if the need won't come up very often).

This issue should probably receive the A-rustbuild, C-feature-request, T-infra, and P-low labels.

[Mark-Simulacrum]

Seeing as I believe we'd like to migrate the compiler over to 2018 edition (and probably std?), I think p-medium might be more accurate.

[ljedrz]

I've started working on this.