Difficulties with integration with source plugins

Consider this minimal source plugin which whenever it encounters the expression `False` checks if `ScopedTypeVariables` are enabled, and if not, throws an error:

```haskell
{-# LANGUAGE CPP #-}

module SayYes (plugin) where

import Control.Monad
import Data.Data
import Data.Generics
import Language.Haskell.TH (Extension(..))

#if __GLASGOW_HASKELL__ < 900
import Bag
import ErrUtils
import GHC
import GhcPlugins
#else
import GHC.Data.Bag
import GHC.Hs
import GHC.Plugins
import GHC.Utils.Error
#endif

plugin :: Plugin
plugin = defaultPlugin { parsedResultAction = \_ _ -> action }

action :: HsParsedModule -> Hsc HsParsedModule
action parsed@HsParsedModule{hpm_module = m} =
(\m' -> parsed {hpm_module = m'}) <$>
gmapM (everywhereM (mkM sayYes)) m

sayYes :: HsExpr GhcPs -> Hsc (HsExpr GhcPs)
sayYes e = do
case e of
HsVar _ (L l x) | occNameString (rdrNameOcc x) == "False" -> do
dynFlags <- getDynFlags
unless (ScopedTypeVariables `xopt` dynFlags) $
issueError l $ text "Everyone needs scoped type variables"
return $ e
_otherwise ->
return $ e

issueError :: SrcSpan -> SDoc -> Hsc a
issueError l errMsg = do
dynFlags <- getDynFlags
throwOneError $
mkErrMsg dynFlags l neverQualify errMsg

issueWarning :: SrcSpan -> SDoc -> Hsc ()
issueWarning l errMsg = do
dynFlags <- getDynFlags
liftIO $ printOrThrowWarnings dynFlags . listToBag . (:[]) $
mkWarnMsg dynFlags l neverQualify errMsg
```

Example use case:

```haskell
-- {-# LANGUAGE ScopedTypeVariables #-}

{-# OPTIONS_GHC -fplugin=SayYes #-}

module Main where

main :: IO ()
main = print False
```

If we compile this on the command line (say, with `cabal`), we get

```
app/Main.hs:8:14: error: Everyone needs scoped type variables
|
8 | main = print False
| ^^^^^
```

unless we enable the pragma. All good.

If we open a project with HLS and open `Main.hs`, there are two orthogonal problems:

1. The error triggers whether or not the language pragma is actually enabled. Curiously, if we use `issueWarning` instead of `issueError`, the opposite is true: we never see the warning at all.
2. The _location_ of the error that is being reported is wrong (`compiler:1:1` instead of the correct location within the source program).

(Tested with ghc 8.8.4, 8.10.7 and 9.0.2. Full example code at https://github.com/well-typed/large-records/tree/edsko/hls-bugreport/hls-plugin-integration .)