smoke-test for async fn with mir-opt-level=0
MIR opt levels heavily influence which MIR transformations run, and we barely test non-default opt levels. I am particularly worried about `async fn` lowering and how it might (not) work when the set of preceding MIR passes changes -- see https://github.com/rust-lang/rust/pull/70073.
This adds some basic smoke testing, where at least a few `async fn` `run-pass` test are ensured to also work with mir-opt-level=0.
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