Auto merge of rust-lang#105363 - WaffleLapkin:thin2win_box_next_argument, r=nnethercote

Shrink `rustc_parse_format::Piece`

This makes both variants closer together in size (previously they were different by 208 bytes -- 16 vs 224). This may make things worse, but it's worth a try.

r? `@nnethercote`

Author: bors