fix r completions in cpp mode

Completions inside of R chunks in C++ mode, e.g.

    /*** R
   
    */

were broken in C++ mode -- there, we were erroneously grabbing the C++ code model, rather than the R code model, and attempting to use that for completions. We now ensure that anything using the R completion frameworks grabs the R code model, rather than the 'primary' code model.

Somewhere along the line (e.g. if we plan on adding any more code models, or more embedded modes) we should consider making `CodeModel` an abstract class with e.g. `RCodeModel`, `CppCodeModel` extending those.