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Now `desugarLocalBind` (formerly `desugarLet`) reasons about * `FunBind`s that * Have no pattern matches (so which aren't functions) * Have a singleton match group with a single GRHS * (which may have guards) * and looks through trivial post-typechecking `AbsBinds` in doing so to pick up the introduced renamings. And desugars to `PmLet` LYG-style guards. Since GRHSs are no longer denoted simply by `NonEmpty PmGRHS`, but also need to carry a `[PmGrd]` for the `PmLet`s from `LocalBind`s, I added `PmGRHSs` to capture that. Since we call out to the desugarer more often, I found that there were superfluous warnings emitted when desugaring e.g. case expressions. Thus, I made sure that we deactivate any warnings in the LYG desugaring steps by the new wrapper function `noCheckDs`. There's a regression test in `T18626`. Fixes #18626.
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