Add use-nullptr transform to cpp11-migrate

This transform converts the usage of null pointer constants (e.g. NULL, 0,
etc.) in legacy C++ code and converts them to use the new C++11 nullptr
keyword.
- Added use-nullptr transform.
- Added C++11 support to the final syntax check. Used ArgumentAdjuster class to
  add -std=c++11 option to the command line options.
- Added tests for use-nullptr transform.
- Added tests that exercises both loop-convert and use-nullptr in the source
  file.

TODO: There's a known bug when using both -loop-convert and -use-nullptr at the
      same time.

Author: Tareq A Siraj <tareq.a.siraj@intel.com>
Reviewers: klimek, gribozavr


git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@173178 91177308-0d34-0410-b5e6-96231b3b80d8