Attach WhileHandling annotations in exceptions thrown from catchSTM handlers

[bgamari]

The catchSTM combinator exposed by GHC.Conc, GHC.Conc.Sync, and the stm package currently does not attach WhileHandling annotations to exceptions thrown from its handler, as described in #202. This functionality was requested in GHC #25365.

I propose that we introduce an annotateSTM combinator to GHC.Conc and GHC.Conc.Sync:

+-- | Execute an 'STM' action, adding the given 'ExceptionContext'
+-- to any thrown synchronous exceptions.
+annotateSTM :: forall e a. ExceptionAnnotation e => e -> STM a -> STM a
+annotateSTM ann (STM io) = STM (catch# io handler)
+  where
+    handler se = raiseIO# (addExceptionContext ann se)

and use it from catchSTM:

 catchSTM (STM m) handler = STM $ catchSTM# m handler'
     where
       handler' e = case fromException e of
-                     Just e' -> unSTM (handler e')
+                     Just e' -> unSTM (annotateSTM (WhileHandling e) (handler e'))
                      Nothing -> raiseIO# e

Implemented in GHC !13408.