C++ includes all the libraries that C89 had, so technically you could safely compile C89 as C++. But you would have to conform to C++'s stringent type system, which requires a cast to and from void* for example, and cannot rely on newer C-only features to do it without warnings or errors.And combining code compiled with a C compiler with a C++ compiler... can you do that? Or am I misunderstanding what you're saying?
Further, good IDEs will let you either compile as C or make a C project, and there are still lone C compilers (such as gcc), so that you should be able to choose the language you want instead of mixing and mashing.
So, yes and no. It depends on what you're doing.