Changing kits, tool chains, release and debug can lead to an reindexing of everything. This is really expensive. But in most cases no symbols will be changed. To compare this sets of flags, defines and include paths it is advantageous to reduce the set to a minimum which is changing the symbols. For debug we even could always set it. Include path outside of the project could be ignored etc.. The target is to reduce the indexing to an reasonable amount. The same technology could be used for project part merging.

Maybe the command line parser of clang could be of help.