ConstPtr loses constness when wrapped in newtype

Consider the following program:

{-# LANGUAGE CApiFFI #-}

module Repro where

import Foreign.C
import Foreign.C.ConstPtr

newtype BadConstPtr a = BadConstPtr (ConstPtr a)

foreign import capi unsafe "foo" c_foo :: IO (ConstPtr CChar)

The generated C stub would contain

const char* ghczuwrapperZC0ZCmainZCReproZCfoo(void) {return foo();}

However, if c_foo is changed to return IO (BadConstPtr CChar), then the C stub becomes

char* ghczuwrapperZC0ZCmainZCReproZCfoo(void) {return foo();}

Whether or not the constness is robust against newtypes is not mentioned either in #22043 (closed) or Foreign.C.ConstPtr haddock. If we decide the constness is only effective when ConstPtr is used verbatim, that's fine, but it should be documented; otherwise it's actually pretty easy to fix the issue, just add logic to constQual in GHC.HsToCore.Foreign.C to expand type synonyms and newtypes.