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Don't prematurely final-zonk PatSyn declarations

sheaf requested to merge sheaf/ghc:T26465 into master

Ticket: #26465 (closed).

This MR makes GHC hold off on the final zonk for pattern synonym declarations, in GHC.Tc.TyCl.PatSyn.tc_patsyn_finish. This accommodates the fact that pattern synonym declarations without a type signature can contain unfilled metavariables, e.g. if the RHS of the pattern synonym involves view-patterns whose type mentions promoted (level 0) metavariables. Just like we do for ordinary function bindings, we should allow these metavariables to be settled later, instead of eagerly performing a final zonk-to-type.

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