C++ PATCH to unary fold-expression semantics

[Jason Merrill]

At Jacksonville the committee adjusted the semantics of C++17 
fold-expressions so that empty expansions are ill-formed for more 
operators.  Since this is a new feature in GCC 6, and the patch is 
trivial, let's update it accordingly.

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