C++ PATCH for c++/85049, ICE with integer_sequence

[Jason Merrill]

In this testcase, we tried to deduce template arguments between
__integer_pack(sizeof...(_Types)) and an empty argument list.  This
breaks, and we shouldn't try anyway, since that's very much a
non-deduced context.  So let's skip over packs that aren't actual
template parameter packs.

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