rustdoc: Fix ICE report
The ICE report in rustdoc was confusing because it was returning an argument parse error:
```
thread 'rustc' panicked at 'aborting due to `-Z treat-err-as-bug=1`', compiler/rustc_errors/src/lib.rs:1212:27
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
error: internal compiler error: unexpected panic
error: Unrecognized option: 'crate-version'
```
This is because the ICE reporter was trying to parse the arguments as rustc, not rustdoc. Since an argument error is a fatal error, it was early-exiting with the argument error due to unwinding.
This changes it to be a more primitive scan of the arguments. The arguments being checked are pretty simple, and only have a small handful of forms that are easy to check for.
It now looks like this:
```
thread 'rustc' panicked at 'aborting due to `-Z treat-err-as-bug=1`', compiler/rustc_errors/src/lib.rs:1212:27
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
error: internal compiler error: unexpected panic
note: the compiler unexpectedly panicked. this is a bug.
note: we would appreciate a bug report: https://github.com/rust-lang/rust/issues/new?labels=C-bug%2C+I-ICE%2C+T-compiler&template=ice.md
note: rustc 1.59.0-dev running on x86_64-apple-darwin
note: compiler flags: --crate-type lib -Z treat-err-as-bug
note: some of the compiler flags provided by cargo are hidden
query stack during panic:
end of query stack
```
It still says `rustc`, but I can live with that.
9298bd8197