Support setting Miri flags in the rustc file via attributes about miri

[RalfJung]

@oli-obk any idea what the best way might be to implement such an attribute? I am imagining something like

#![miri::flag("-Zmiri-tree-borrows")]

Usually this would be gated by cfg_attr(miri, ...).

In a lib crate this has no effect (we should likely lint against that), but in a binary crate this makes Miri act as-if those flags were passes on the command line.

Or should this maybe even be a more general rustc feature?

[oli-obk]

yea I've wanted this as a general rustc feature before, but it can't really work because some flags are processed before the source file is even opened.

I think I would prefer just allowing miri flags to also be used via native attributes like #[miri::tree_borrows]. We can set up the infrastructure to not duplicate the logic between attributes and command line flags