Always produce sub-obligations when using cached projection result
See https://github.com/rust-lang/rust/issues/85360
When we skip adding the sub-obligations to the `obligation` list, we can affect whether or not the final result is `EvaluatedToOk` or `EvaluatedToOkModuloObligations`. This creates problems for incremental compilation, since the projection cache is untracked shared state.
To solve this issue, we unconditionally process the sub-obligations. Surprisingly, this is a slight performance *win* in many cases.
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