Statistic: Add machine parseable json output

- We lacked a short unique identifier for a statistics, so I renamed the
  current "Name" field that just contained the DEBUG_TYPE name of the
  current file to DebugType and added a new "Name" field that contains
  the C++ identifier of the statistic variable.
- Add the -stats-json option which outputs statistics in json format.

Differential Revision: http://reviews.llvm.org/D20995

llvm-svn: 272826