Split `SynTyCon` into `SynonymTyCon` and `FamilyTyCon`

As part of my work on #6018 (closed) I added new fields to SynTyCon data constructor of TyCon type. These fields only make sense for type families because we don't trace injectivity information for type synonyms. SynTyCon already has some fields that only make sense for type families. Richard suggests that SynTyCon is split into two separate constructors. With my implementation of injective type families this makes even more changes. I'm going to make that change in HEAD Real Soon Now so that I can rebase my work on top of it.

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Trac field	Value
Version	7.9
Type	Task
TypeOfFailure	OtherFailure
Priority	low
Resolution	Unresolved
Component	Compiler
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