Fix ICE report flags display.
#92310 made some changes to the ICE report that displays the rustc flags, but it introduced a bug where a flag like `-Z incremental-verify-ich=yes` was being treated as-if it was `-Cincremental`. This corrupted the output and made it confusing. The cause was using `starts_with` instead of properly splitting the option.
For example, with the command like `rustc foo.rs -Cincremental=/tmp/a -Zincremental-verify-ich=yes --crate-type lib` would previously look like:
```
note: compiler flags: -C incremental -Z incremental --crate-type lib
```
It now looks like:
```
note: compiler flags: -C incremental=[REDACTED] -Z incremental-verify-ich=yes --crate-type lib
```
I added a `[REDACTED]` marker for `-Cincremental` so it is a little less confusing that a value has been removed.
Fixes#101588
Matthias Krüger
3ddb0482a3