Improve the inline_fluent! macro

[JonathanBrouwer]

For #151366

This PR turns inline_fluent! into a proc macro, so we can run validation on the messages in this macro :)
I started a thread here because I don't like the name of the macro, but that's for a future PR: #t-compiler > Bikeshed the new `inline_fluent!` macro

[JonathanBrouwer]

r? @jdonszelmann

[rustbot]

rustc_errors::translation was changed

cc @davidtwco, @TaKO8Ki

rustc_macros::diagnostics was changed

cc @davidtwco, @TaKO8Ki

Some changes occurred to the CTFE machinery

cc @RalfJung, @oli-obk, @lcnr

[JonathanBrouwer]

Don't merge together with #152276, these will soft-conflict

[jdonszelmann]

don't merge, #152276 goes first

[rustbot]

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

Some changes occurred to the CTFE / Miri interpreter

cc @rust-lang/miri

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[JonathanBrouwer]

Soft conflict is now resolved, safe to merge :)
@jdonszelmann I don't think you reviewed this yet, right?
The new diff is only reformatting of messages due to #152276