Allow generation of TTH syntax with TH

In other words allow generation of typed splices and brackets with
Untyped Template Haskell.

That is useful in cases where a library is build with TTH in mind,
but we still want to generate some auxiliary declarations,
where TTH cannot help us, but untyped TH can.
Such example is e.g. `staged-sop` which works with TTH,
but we would like to derive `Generic` declarations with TH.

An alternative approach is to use `unsafeCodeCoerce`, but then the
derived `Generic` instances would be type-checked only at use sites,
i.e. much later. Also `-ddump-splices` output is quite ugly:
user-written instances would use TTH brackets, not `unsafeCodeCoerce`.

This commit doesn't allow generating of untyped template splices
and brackets with untyped TH, as I don't know why one would want to do
that (instead of merging the splices, e.g.)