Provide the ability to add framework paths as system include directories (-iframework)

We should implement -iframework through a systemFrameworkPaths variable. In summary:

includePaths: normal include paths, -I
frameworkPaths: framework include paths, -F
systemIncludePaths: system include paths, -isystem
systemFrameworkPaths: system framework paths, -iframework

       -iframeworkdir
           Like -F except the directory is a treated as a system directory.
           The main effect is to not warn about constructs contained within
           header files found via dir.