Refactoring transform-specific options
    
Making the user null macros command-line option visible to the
UseNullptrTransform class instead of being visible only to the match callback.



git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@192905 91177308-0d34-0410-b5e6-96231b3b80d8