Control validity-checking of type synonym applications more carefully
Trac #16059 shows that when validity checking applications of type synonyms, GHC sometimes wasn't checking the expanded type enough. We must be careful, however, since checking both the expanded type as well as the arguments to the type synonym can lead to exponential blowup (see https://ghc.haskell.org/trac/ghc/ticket/16059#comment:4). Nor can we omit checking either the expanded type or the argument for correctness reasons. The solution here is to introduce a new `ExpandMode` data type that is plumbed through all of the type-validity-checking functions in `TcValidity`. `ExpandMode` dictates whether we only check the expanded type (`Expand`), only check the arguments (`NoExpand), or both (`Both`). Importantly, if we check `Both` in the function for validity checking type synonym applications, then we switch to `NoExpand` when checking the arguments so as to avoid exponential blowup. See `Note [Correctness and performance of type synonym validity checking]` for the full story.
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