Run compiler doctests in CI

[jyn514]

Right now these are only ever run locally, which means they often break. We should run these in CI.

See #99144 (comment) for mentoring instructions.

Originally posted by @jyn514 in #95994 (comment)

[jyn514]

Maybe x86_64-gnu-aux is a good place? That has some existing "misc tests", and already builds stage 2 rustc, so shouldn't slow down the builder too much.

[Mark-Simulacrum]

Seems okay. Any test builder under 2 hours right now on Linux is probably fine.

[reez12g]

@rustbot claim

[jyn514]

@reez12g see #99040 for an example of something similar

[reez12g]

The remaining crates that need to be fixed are

• rustc_ast_lowering: claimed by @GentBinaku
• rustc_builtin_macros
• rustc_expand
• rustc_infer: claimed by @CarsonV
• rustc_middle
• rustc_query_impl

Here's steps to fix these.

1. Remove doctest=false in the target Calgo.toml.
   e.g. this
2. Run the test locally.
   e.g. python3 x.py test compiler/rustc_ast_lowering --stage 0
3. Fix the errors.

@jyn514
I couldn't edit the description, so I left it as a comment.

[nakatomotoi]

@rustbot claim