Deduplicate rustc-ap-syntax in the RLS

[alexcrichton]

As of #52919 we'll be updating to a version of the RLS which depends on racer which depends on rustc-ap-syntax. Unfortunately we're bringing in two different versions of rustc-ap-syntax, 209 and 211, right now.

These should be deduplicated as they take a good deal of time to build on CI, and then we should also add a tidy check which ensures that there's only one version of rustc-ap-syntax in our workspace.

cc @nrc

[alexcrichton]

I've sent a PR to update racer as part of racer-rust/racer#915

[nrc]

This is fixed by #53141