Auto merge of #128200 - estebank:normalize-whitespace, r=pnkfelix

Change output normalization logic to be linear against size of output

Modify the rendered output normalization routine to scan each character *once* and construct a `String` to be printed out to the terminal *once*, instead of using `String::replace` in a loop multiple times. The output doesn't change, but the time spent to prepare a diagnostic is now faster (or rather, closer to what it was before #127528).

Author: bors