Highlight C types from tags in C++ code

Useful for when using, for example, GTK+, SDL, and other C-only APIs
in files with the C++ filetype. Before only the symbol auto-completion
worked, but not the highlighting of type names.

TODO: figure out why taking something like SDL.c.tags and making a copy
as SDL.cpp.tags doesn't work, once that's fixed, this extra condition
could probably be removed.