Update doctest-0.20.0 patch to accommodate "hi haddock: Lex and store haddock docs in interface files"

After commit ghc@b91798be (hi haddock: Lex and store haddock docs in interface files), the doctest-0.20.0 patch no longer compiles. Here is an example of the error message from a recent build-master CI job:

[10 of 16] Compiling Extract          ( src/Extract.hs, dist/build/Extract.o, dist/build/Extract.dyn_o )
src/Extract.hs:226:56: error:
    Variable not in scope: unpackHDS :: HsDoc GhcPs -> String
    Suggested fix: Perhaps use ‘unpackHDSC’ (imported from GHC)
    |
226 | docStringsFromModule mod = map (fmap (toLocated . fmap unpackHDS)) docs
    |                                                        ^^^^^^^^^
src/Extract.hs:231:26: error:
    • Couldn't match type: GenLocated SrcSpan (HsDoc GhcPs)
                     with: WithHsDocIdentifiers HsDocString GhcPs
      Expected: [(Maybe String, LHsDoc GhcPs)]
        Actual: [(Maybe String, GenLocated SrcSpan (LHsDoc GhcPs))]
    • In the first argument of ‘(++)’, namely ‘exports’
      In the second argument of ‘(++)’, namely ‘exports ++ decls’
      In the expression: header ++ exports ++ decls
    |
231 |     docs     = header ++ exports ++ decls
    |                          ^^^^^^^
src/Extract.hs:231:37: error:
    • Couldn't match type ‘HsDocString’
                     with ‘WithHsDocIdentifiers HsDocString GhcPs’
      Expected: [(Maybe String, LHsDoc GhcPs)]
        Actual: [(Maybe String, LHsDocString)]
    • In the second argument of ‘(++)’, namely ‘decls’
      In the second argument of ‘(++)’, namely ‘exports ++ decls’
      In the expression: header ++ exports ++ decls
    |
231 |     docs     = header ++ exports ++ decls
    |                                     ^^^^^
src/Extract.hs:318:31: error:
    • Expecting one more argument to ‘DocDecl’
      Expected a type, but ‘DocDecl’ has kind ‘* -> *’
    • In the type signature:
        fromDocDecl :: SrcSpan -> DocDecl -> (Maybe String, LHsDocString)
      In an equation for ‘extractDocStrings’:
          extractDocStrings
            = everythingBut
                (++)
                (([], False) `mkQ` fromLHsDecl `extQ` fromLDocDecl
                   `extQ` fromLHsDocString)
            where
                fromLHsDecl :: Selector (LHsDecl GhcPs)
                fromLHsDecl (L loc decl)
                  = case decl of
                      DocD _ x -> select (fromDocDecl (locA loc) x)
                      _ -> ...
                fromLDocDecl :: Selector (LDocDecl GhcPs)
                fromLDocDecl (L loc x) = select (fromDocDecl (locA loc) x)
                ....
    |
318 |     fromDocDecl :: SrcSpan -> DocDecl -> (Maybe String, LHsDocString)
    |                               ^^^^^^^

The API changed pretty substantially in that commit, so it may take a bit of work to update the doctest-0.20.0 patch accordingly. Depending on how involved the changes are, we may want to consider adding doctest to the list of expected failures in the meantime.