Added lf_combine_deadline_and_level function for use by code generator

[edwardalee]

This PR suppresses a warning (below) and consolidates the mechanism that is used to combine a deadline with a level into a single C function. As usual, there is a companion PR in lingua-franca. The warning was as follows:

/Users/edwardlee/git/epoch/org.lflang.ui/src/org/lflang/ui/wizard/templates/c/src-gen/Pipeline/Pipeline.c:562:91: warning: signed shift result (0x7FFFFFFFFFFFFFFF0000) requires 80 bits to represent, but 'long' only has 64 bits [-Wshift-overflow]
            pipeline_rp_self[pipeline_rp_i]->_lf__reaction_0.index = (9223372036854775807 << 16) | pipeline_rp_reaction_1_levels[pipeline_rp_i];
                                                                      ~~~~~~~~~~~~~~~~~~~ ^  ~~
1 warning generated.

[lhstrh]

👍