In 79556, we try to deduce an auto type from a dependent initializer
with null TREE_TYPE, which doesn't work; fixed by catching that case
in do_auto_deduction.

In 79549, we try to tsubst into the type of a NONTYPE_ARGUMENT_PACK,
which doesn't make sense for an auto parameter pack; in fact, it
doesn't make sense for the argument pack to have a type at all.  For
GCC 7 I'm fixing this by leaving the auto type in place; for GCC 8
we'll do away with TREE_TYPE on all NONTYPE_ARGUMENT_PACKs.

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