C++ PATCH for c++/55220 (ICE with non-deducible parameter pack)

[Jason Merrill]

This testcase is well-formed, even though it doesn't do what the user 
was trying to accomplish.  In Tuple<Head..., T> the use of a pack 
expansion not at the end of the list makes the whole argument list a 
non-deduced context, so we should get out early before it confuses us.

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