For headers, also use source-file include paths and defines.
Review Request #121649 - Created Dec. 23, 2014 and discarded - Latest diff uploaded
We use the existing find-buddy-document architecture to find a source file corresponding to the current header file. If we find a match that also has a project file, we ask it for its include files. This fixes an issue, where when editing a header file Clang would never find the include paths. The reason is that in e.g. CMake, only the .cpp files have include paths, but not the .h files.