Rollup merge of #154745 - chenyukang:yukang-fix-span-api, r=nnethercote

Replace span_look_ahead with span_followed_by

While reviewing that PR rust-lang/rust#154703 (comment), I found that magic number 100, let's remove it, and seems `span_followed_by` is a better name.

Author: JonathanBrouwer