Rollup merge of #102639 - nnethercote:improve-spans-splitting, r=Aaron1011

Improve spans when splitting multi-char operator tokens for proc macros.

When a two-char (or three-char) operator token is split into single-char operator tokens before being passed to a proc macro, the single-char tokens are given the original span of length two (or three). This PR gives them more accurate spans.

r? `@Aaron1011`
cc `@petrochenkov`