[analyzer] Remove superquadratic behaviour from DataflowWorklist

The class DataflowWorklist internally maintains a sorted list of pointers to CFGBlock
and the method enqueuePredecessors has to call sortWorklist to maintain the invariant. 
The implementation based on vector + sort works well for small sizes 
but gets infeasible for relatively large sizes. In particular the issue takes place 
for some cryptographic libraries which use code generation. 
The diff replaces vector + sort with priority queue.
For one of the implementations of AES this patch reduces 
the time for analysis from 204 seconds to 8 seconds.

Test plan: make -j8 check-clang

Differential revision: https://reviews.llvm.org/D25503

llvm-svn: 284166