Typefamily instance support in boot files

[fxdpntthm]

In general it may not be possible, but we could reduce a lot of code change if we could enable it for wf type families.
https://github.com/IaFP/ghc/blob/wip/party-ctrs-ghc-9/compiler/GHC/Tc/Module.hs#L855

[fxdpntthm]

It is annoying to deal with these kinds of errors:

compiler/GHC/Hs/Pat.hs:288:1: error:
    Identifier ‘pprLPat’ has conflicting definitions in the module
    and its hs-boot file
    Main module: pprLPat ::
                   forall (p :: Pass).
                   ($wf'XOverLit (GhcPass p), GhcPass @ p, WFT (XOverLit (GhcPass p)),
                    $wf'XOverLit (GhcPass (NoGhcTcPass p)), GhcPass @ NoGhcTcPass p,
                    WFT (XOverLit (GhcPass (NoGhcTcPass p))), $wf'Anno (HsExpr GhcRn),
                    HsExpr @ GhcRn, WFT (Anno (HsExpr GhcRn)),
                    $wf'Anno (HsExpr (GhcPass p)), HsExpr @ GhcPass p, GhcPass @ p,
                    WFT (Anno (HsExpr (GhcPass p))), $wf'Anno (IdGhcP p),
                    $wf'Anno (IdGhcP (NoGhcTcPass p)), OutputableBndrId p) =>
                   LPat (GhcPass p) -> SDoc
    Boot file:   pprLPat ::
                   forall (p :: Pass).
                   ($wf'Anno (Pat (GhcPass p)), $wf'XOverLit (GhcPass p), GhcPass @ p,
                    WFT (XOverLit (GhcPass p)), $wf'XOverLit (GhcPass (NoGhcTcPass p)),
                    GhcPass @ NoGhcTcPass p, WFT (XOverLit (GhcPass (NoGhcTcPass p))),
                    $wf'Anno (HsExpr GhcRn), HsExpr @ GhcRn, WFT (Anno (HsExpr GhcRn)),
                    $wf'Anno (HsExpr (GhcPass p)), HsExpr @ GhcPass p, GhcPass @ p,
                    WFT (Anno (HsExpr (GhcPass p))), $wf'Anno (IdGhcP p),
                    $wf'Anno (IdGhcP (NoGhcTcPass p)), OutputableBndrId p) =>
                   LPat (GhcPass p) -> SDoc