add guard expression to system function

[Stephen Siegel]

When declaring a library system function with #pragma TASS system there also needs to be a way to declare a guard expression for the system function. Right now, system functions are treated like regular function calls, so the guard is always "true". However some system functions require non-trivial guards. Example: MPI_Send or MPI_Recv.

How about an optional form of the pragma like this:

#pragma TASS system guard <TASS expression>

?

[Stephen Siegel]

I have added the ability to specify guards at the model level. It now requires modifications to the front end. The grammar, TreeBuilder, and ModelBuilder need to be modified to add support for the optional guard form. I put a TODO comment at the point in the Model Builder where the change needs to take place.

[zirkel]

Front end support has been added, and the guard is being correctly set. However, currently TASS will verify a program as correct even if there is a system function whose guard is never true. See examples/fooLibraryUse/guardedSystemFunctionFail.c