C++ PATCH for c++/64095, unnamed generic lambda parameter pack

[Jason Merrill]

In this testcase, we have (auto...), a function parameter pack without
a declarator.  Because there is no declarator, we were never hitting
the code for turning the auto type into a template parameter pack, so
the ... ended up being parsed as a C-style trailing ellipsis.

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