[analyzer] Fix JSON dumps for ExplodedNodes

Summary:
- Now we could see the `has_report` property in `trim-egraph` mode.
- This patch also removes the trailing comma after each node.

Reviewers: NoQ

Reviewed By: NoQ

Subscribers: xazax.hun, baloghadamsoftware, szepet, a.sidorin,
             mikhail.ramalho, Szelethus, donat.nagy, dkrupp, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D63436

llvm-svn: 364193