Don't catch async exceptions when evaluating Template Haskell

This is a fix for a 12 year old TODO.

-- XXX We shouldn't be catching everything, e.g. timeouts
tryAllM :: IOEnv env r -> IOEnv env (Either SomeException r) 

The only place where tryAllM is used is while evaluating Template Haskell code, in runMeta' (GHC.Tc.Gen.Splice)

        ; either_tval <- tryAllM $
                         setSrcSpan expr_span $ -- Set the span so that qLocation can
                                                -- see where this splice is
             do { mb_result <- run_and_convert expr_span hval
                ; case mb_result of
                    Left err     -> failWithTc err
                    Right result -> do { traceTc "Got HsSyn result:" (ppr_hs result)
                                       ; return $! result } }

ghcide often shows a spurious compiler error/diagnostic that looks like "Exception when trying to run compile-time code" since it uses asynchronous exceptions to stop no-longer valid typechecks when the user types some characters in their editor while some TH is being evaluated.

Fixes #19410 (closed)