FIX #2500: Don't log warnings in getHeader

compiler/main/HeaderInfo.hs

@@ -59,8 +59,10 @@ getImports dflags buf filename source_filename = do
   case unP parseHeader (mkPState buf loc dflags) of
     PFailed span err -> parseError span err
     POk pst rdr_module -> do
-      let ms@(warns, errs) = getMessages pst
-      logWarnings warns
+      let ms@(_warns, errs) = getMessages pst
+      -- don't log warnings: they'll be reported when we parse the file
+      -- for real.  See #2500.
+      -- logWarnings warns
       if errorsFound dflags ms
         then liftIO $ throwIO $ mkSrcErr errs
         else