Do not increase recursion depth for coroutine witness and rigid opaques when proving auto traits

[adwinwhite]

With deeply nested async calls, we can easily overflow evaluating auto trait goals as shown in #159228.
Users have no choice but increase the per-crate recursion_limit which is bad for compilation time/RSS. And downstream users may encounter the same warnings when calling library async functions.

We mitigate that by no longer increasing recursion depth for coroutine witness and rigid opaques when proving auto traits. See the comments for why it's okay to do so.
This fix actually makes the required depth a third of what it was before for async calls.

I've checked locally that bors and triagebot no longer have FCWs.

r? lcnr