setSessionDynamicFlags to prevent ghc9 from crashing

The only way to set the dynamic linker is with the function `setSessionDynFlags`
so we call it with the result from `getSessionDynFlags` to give it a
(hopefully sensible) argument.

See also this commit:
https://gitlab.haskell.org/ghc/ghc/commit/18757cab04c5c5c48eaceea19469d4811c5d0371

ghcide/session-loader/Development/IDE/Session.hs

@@ -525,12 +525,15 @@ cradleToOptsAndLibDir cradle file = do
 
 emptyHscEnv :: IORef NameCache -> FilePath -> IO HscEnv
 emptyHscEnv nc libDir = do
+#if MIN_VERSION_ghc(9,0,0)
+    -- Without the seemingly redundant `getSessionDynFlags >>= setSessionDynFlags`
+    -- the line `initDynLinker env` would crash with the errror:
+    -- `Couldn't find a target code interpreter. Try with -fexternal-interpreter`
+    env <- runGhc (Just libDir) $ getSessionDynFlags >>= setSessionDynFlags >> getSession
+#else
     env <- runGhc (Just libDir) getSession
-#if !MIN_VERSION_ghc(9,0,0)
-    -- This causes ghc9 to crash with the error:
-    -- Couldn't find a target code interpreter. Try with -fexternal-interpreter
-    initDynLinker env
 #endif
+    initDynLinker env
     pure $ setNameCache nc env{ hsc_dflags = (hsc_dflags env){useUnicode = True } }
 
 data TargetDetails = TargetDetails