[clang][dataflow] Make limit on fixpoint-algorithm iterations proportional to size of CFG.

Currently, the maximum number of iterations of the loop for finding the fixpoint
of the dataflow analysis is set at 2^16. When things go wrong in an analysis,
this can be far too large.  This patch changes the limit to be proportional to
the size of the CFG, which will generally be far smaller than 2^16 (while still
maintaining 2^16 as the absolute limit).

Differential Revision: https://reviews.llvm.org/D126316