Auto merge of #137348 - compiler-errors:span-trim, r=estebank

More sophisticated span trimming for suggestions

Previously #136958 only cared about prefixes or suffixes. Now it detects more cases where a suggestion is "sandwiched" by unchanged code on the left or the right. Would be cool if we could detect several insertions, like `ACE` going to `ABCDE`, extracting `B` and `D`, but that seems unwieldy.

r? `@estebank`

Author: bors