Currently, `early_error` and `early_warn` in `librustc::session` always
use `ColorConfig::Auto`. Modify them to follow the color configuration
set by the `--color` option.

As colored output is also printed during the early stage, parsing the
`--color` option should be done as early as possible. However, there are
still some cases when the output needs to be colored before knowing the
exact color settings. In these cases, it will be defaulted to
`ColorConfig::Auto`, which is the same as before.

Fixes #27879.