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.
