Auto merge of #110407 - Nilstrieb:fluent-macro, r=davidtwco

Add `rustc_fluent_macro` to decouple fluent from `rustc_macros`

Fluent, with all the icu4x it brings in, takes quite some time to compile. `fluent_messages!` is only needed in further downstream rustc crates, but is blocking more upstream crates like `rustc_index`. By splitting it out, we allow `rustc_macros` to be compiled earlier, which speeds up `x check compiler` by about 5 seconds (and even more after the needless dependency on `serde_json` is removed from `rustc_data_structures`).

Author: bors