Add option to print preprocessed P4 and continue with compilation

[kfcripps]

When the -E option is provided, the compiler prints the contents of the preprocessed P4 file, and exits. It would be nice if there existed an option to do the same (and maybe print the preprocessed P4 to some output file in the --dump directory instead of to stdout), but continue with compilation instead of exiting. Is there any reason to not add such an option?

[asl]

Normally such option is called --save-temps for C/C++ compilers

[vgurevich]

Actually, Tofino compiler does it already by default, but this is done by the compiler driver (p4c or bf-p4c) (by invoking CPP with the proper parameters) and not by the backend (p4c-barefoot) itself.