While rewriting the template parameters of a member template
constructor to generate the corresponding deduction guide, we tsubst
the parms twice.  In this bug, on the first pass we ended up lowering
the level of the 'auto' TEMPLATE_TYPE_PARM to 1, so the second pass
blows up.  This is very similar to the situation that I already
introduced tf_partial for, so let's use it here as well.

Tested x86_64-pc-linux-gnu, applying to trunk.