Need -fno-print-explicit-runtime-reps to work on IfaceType, else RuntimeRep leaks

After #11549 (closed), we are suppose not to show RuntimeRep to users, when -fno-print-explicit-runtime-reps is on. But we do:

:info ($)
Prelude> :i ($)
($) ::  forall (r :: GHC.Types.RuntimeRep) a (b :: TYPE r).
        (a -> b) -> a -> b
  	-- Defined in ‘GHC.Base’

This happens because the runtime-rep-suppression happens on Type, but for :info we use pprInfo which uses tyThingToIfaceDecl and then prints it. And tyThingToIfaceDecl doesn't know about the -fprint-explicit-runtime-reps flag; and I'd like to keep it that way.

We want to change pprType to convert to IfaceType and then print anyway.

I suppose that this means that the pretty-printer for IfaceType needs to know about this runtime-rep suppression. That's a bit tiresome, but not impossible, especially since there is no shadowing to worry about.

Trac metadata

Trac field	Value
Version	7.10.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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