[analyzer][AST] print() JSONify: Stmt implementation

Summary:
This patch also adds a function called `JsonFormat()` which:
- Flattens the string so removes the new-lines.
- Escapes double quotes.

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/D62494

llvm-svn: 362000