[JSONExporter] Replace bundled Jsoncpp with llvm/Support/JSON.h. NFC.

Differential Revision: https://reviews.llvm.org/D49950

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@338461 91177308-0d34-0410-b5e6-96231b3b80d8
22 files changed