Allow for switching parse context when a file is used in differnt projects in the same session.

Note: I'm not using SUBDIRS because of legacy structure of our repo.

When I have a single project with both C and C++ source files I can switch between multiple parse contexts for the headers. However, if my project only has one type of source files, I will not get an option to switch contexts even though the same file is being used in other projects that I have opened in the same session.

My story: I have to write a library that must be usable from both C and C+. In my current session I have two projects: a library itself, which contains only C+ files (with a lot of extern "C" functions) and a pure C project to see how it works there.