Allow users to specify NULL like macros to be replaced

-use-nullptr only replaced macro named NULL and ignored any user defined
macros that behaved like NULL. This patch introduces -user-null-macros
command line option to let users specify their custom NULL like macros.

- Added a -user-null-macros command line option that takes a
  comma-separated list of user-defined macros to be replaced when using
  the -use-nullptr transform.
- Added documentation.
- Updated testcase to reflect current behavior.
- Whitespace fixes.

Reviewers: revane, klimek, gribozavr



git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@178243 91177308-0d34-0410-b5e6-96231b3b80d8