[analyzer] print() JSONify: getNodeLabel implementation

Summary: This patch also rewrites the ProgramPoint printing.

Reviewers: NoQ, xazax.hun, ravikandhadai, baloghadamsoftware, Szelethus

Reviewed By: NoQ

Subscribers: cfe-commits, szepet, rnkovacs, a.sidorin, mikhail.ramalho,
             donat.nagy, dkrupp

Tags: #clang

Differential Revision: https://reviews.llvm.org/D62346

llvm-svn: 361997