Add 'manual' support for adding PCH(s) to the Clang Code Model

Hello there,

it would be nice to have precompiled headers manual support for non-qt projects such as Makefile etc, because at the moment I'm only getting errors in my headers files, even if my project builds fine without any warnings.

https://github.com/qt-creator/qt-creator/search?q=setPreCompiledHeaders

From what I see only qbs, qmake and cmake have atleast a way to set the precompiled headers, but what if i'm not using these build systems?

If it's possible I would like to have an option somewhere in the projects settings to manually add the precompiled headers for the Clang Code Model, so it can parse the others headers fine.

If needed, I can try to implement the support myself into the IDE as a pull request.