C++ PATCH to implement fold-expressions

[Jason Merrill]

Back in January Andrew mostly implemented C++1z fold-expressions 
(https://isocpp.org/files/papers/n4295.html), but the patch broke 
bootstrap.  I've fixed it up now and am committing it to the trunk.

Andrew: The main change I made was to drop tentative parsing in favor of 
scanning the tokens ahead looking for an ellipsis next to a 
fold-operator.  I also tweaked some diagnostics, fixed handling of 
dependent expressions with no TREE_TYPE, and corrected empty fold of modops.

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