[AST] Add TableGen for StmtDataCollectors

Summary:
This adds an option "-gen-clang-data-collectors" to the Clang TableGen
that is used to generate StmtDataCollectors.inc.

Reviewers: arphaman, teemperor!

Subscribers: mgorny, cfe-commits

Differential Revision: https://reviews.llvm.org/D37383

llvm-svn: 312634