Process pch++ json compilation database

When a project contains a C++ or Objective-c++ file, Xcode will generate two version of precompiled header files.
for C/ObjectiveC , Xcode uses ProcessPCH command, for C++/Objective-C++ Xcode uses ProcessPCH++ command.
Currently, json-compilation-database reporter only checks ProcessPCH not for ProcessPCH++, then any project contains C++ or Objective-C++ code, will produce empty compilation codebase. 

The pull request adds support ProcessPCH++ command, and added a new unit test for it.