Function get_backend_from_raw_matches 

pub(crate) fn get_backend_from_raw_matches(
    early_dcx: &EarlyDiagCtxt,
    matches: &Matches,
) -> Box<dyn CodegenBackend>

Get the codegen backend based on the raw Matches.

rustc -vV and rustc -Cpasses=list need to get the codegen backend before we have parsed all arguments and created a Session. This function reads -Zcodegen-backend, --target and --sysroot without validating any other arguments and loads the codegen backend based on these arguments.