Clean emitted diagnostics when `reset_err_count` is called.
When external tools like `rustfmt` calls to `reset_err_count` for handler
reusing, it will set the error count on the handler to 0, but since
https://github.com/rust-lang/rust/pull/47146 the handler will contain
status that will prevent the error count to be bumped if this handler is
reused.
This caused `rustfmt` idempotency tests to fail:
https://github.com/rust-lang-nursery/rustfmt/issues/2338
Fixes: https://github.com/rust-lang-nursery/rustfmt/issues/2338