This commit replaces unstructured SDoc arguments in error message constructors
with uses of the following two datatypes:

  - SigLike: for different kinds of signatures (e.g. standalone kind signatures,
    fixity signatures, COMPLETE pragmas, etc)
  - Subordinate: for class methods, associated types, and record fields

The following error message constructors now no longer have any SDocs in them:

  - TcRnIllegalBuiltinSyntax: SDoc -> SigLike
  - MissingBinding: SDoc -> SigLike
  - UnknownSubordinate: SDoc -> (Name, Subordinate)
  - SuggestMoveToDeclarationSite: SDoc -> SigLike