Add #if to temporarily omit PRIF/Caffeine code using assumed rank

[bonachea]

flang currently lacks support for assumed-rank dummy arguments like array(..). These are currently required by the following PRIF procedures, and their corresponding Caffeine implementation:

1. prif_put (non-_raw version only)
2. prif_get (non-_raw version only)
3. prif_co_* (all collectives)

This issue suggests creating a temporary (and not officially supported) "dialect" of PRIF and Caffeine, gated by a preprocessor symbol, that entirely omits these features and their corresponding implementation.

To be concrete, this could look something like a -DMISSING_ASSUMED_RANK preprocessor flag added to FPM's C and Fortran flags, and corresponding #if !MISSING_ASSUMED_RANK guards around the declarations and code corresponding to the features above.

Having this in-place should enable us to add flang into CI and test future PRIF PRs submitted against LLVM, which makes this high-priority.