[clang][dataflow] Add limits to size of modeled data structures in environment.

Adds two new parameters to control the size of data structures modeled in the environment: # of values and depth of data structure.  The environment already prevents creation of recursive data structures, but that was insufficient in practice. Very large structs still ground the analysis to a halt.  These new parameters allow tuning the size more effectively.

In this patch, the parameters are set as internal constants. We leave to a future patch to make these proper model parameters.

Differential Revision: https://reviews.llvm.org/D120510