Add StructuredData plugin type; showcase with new DarwinLog feature

Take 2, with missing cmake line fixed.  Build tested on
Ubuntu 14.04 with clang-3.6.

See docs/structured_data/StructuredDataPlugins.md for details.

differential review: https://reviews.llvm.org/D22976

reviewers: clayborg, jingham
llvm-svn: 279202