Rollup merge of #142708 - Darksonn:location-len-without-nul, r=Mark-Simulacrum

Do not include NUL-terminator in computed length

This PR contains just the first commit of rust-lang/rust#142579 which changes it so that the string length stored in the `Location` is the length of the `&str` rather than the length of the `&CStr`. Since most users will want the `&str` length, it seems better to optimize for that use-case.

There should be no visible changes in the behavior or API.

Author: matthiaskrgr