Rollup merge of #92460 - dwrensha:fix-92267, r=petrochenkov

[rustc_builtin_macros] add indices to format_foreign::printf::Substitution::Escape

Fixes #92267.

The problem was that the escape string "%%" does not need to appear at the very beginning of the format string, but
the iterator implementation assumed that it did.

The solution follows the pattern used by `format_foregin::shell::Subtitution::Escape`: https://github.com/rust-lang/rust/blob/8ed935e92dfb09ae388344b12284bf5110cf9265/compiler/rustc_builtin_macros/src/format_foreign.rs#L629