refactor rustc_on_unimplemented's filtering

[mejrs]

Previously when you had a

pub struct Directive {
    pub is_rustc_attr: bool,
    pub condition: Option<OnUnimplementedCondition>,
    pub subcommands: ThinVec<Directive>,
    pub message: Option<(Span, FormatString)>,
    ...
}

that condition would control the emission of the message, label, notes etc. I've changed that to

pub struct Directive {
    pub is_rustc_attr: bool,
    pub filters: ThinVec<(Filter, Directive)>,
    pub message: Option<(Span, FormatString)>,
    ...

so that the message etc is always emitted, and there's a vec of tuples with (filter, directive) where the filter controls whether that directive is even emitted, which i think is much clearer. That also makes it easier to not have to do the reverse iteration thing and this makes it so that notes are emitted in declaration order (with nonfiltered options always last).

The rename is because I plan on making it available to other diagnostic attributes at some point (very wip) so OnUnimplementedCondition and the like would have to be renamed anyway.

[rustbot]

Some changes occurred in compiler/rustc_hir/src/attrs

cc @jdonszelmann, @JonathanBrouwer

Some changes occurred in compiler/rustc_attr_parsing

cc @jdonszelmann, @JonathanBrouwer

[rustbot]

r? @dingxiangfei2009

rustbot has assigned @dingxiangfei2009.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

Why was this reviewer chosen?

The reviewer was selected based on:

• Owners of files modified in this PR: compiler
• compiler expanded to 73 candidates
• Random selection from 20 candidates

[jdonszelmann]

r? jdonszelmann (I don't mind taking this one for tomorrow :)