Update parser initialization to match rust-lang/rust#49387

[jsgf]

I changed the API for ParseSess::with_span_handler() in rust-lang/rust#49387, but I'm not sure how to coordinate cross-repo changes like this.

[topecongiro]

@jsgf Thank you for the PR! So rustfmt does not directly rely on libsyntax anymore, instead we rely on rustc-ap-syntax crate, which is basically a published version of libsyntax. Therefore I think that you need not this PR to get rust-lang/rust#49387 merged.

I am closing this for now, though we need to update rustfmt once rust-lang/rust#49387 gets merged and rustc-ap-syntax includes it in the future.

[jsgf]

Ah ok. I made this change so that the rustfmt git submodule would build within the overall rust toolchain build.