[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:🐚:Subtitution::Escape`: 8ed935e92d/compiler/rustc_builtin_macros/src/format_foreign.rs (L629)
913bc8644a