Return `ExitCode` from `rustc_driver::main` instead of calling `process::exit`
This makes rustc simply return an exit code from main rather than calling `std::process::exit` with an exit code. This means that drops run normally and the process exits cleanly. This is similar to what happens when an ICE occurs (due to being a panic that's caught by std's `lang_start`).
Also instead of hard coding success and failure codes this uses `ExitCode::SUCCESS` and `ExitCode::FAILURE`, which in turn effectively uses `libc::EXIT_SUCCESS` and `libc::EXIT_FAILURE` (via std). These are `0` and `1` respectively for all currently supported host platforms so it doesn't actually change the exit code.
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