[JSON] Remove Utility/JSON.{h|cpp}

This patch is the final step in my quest to get rid of the JSON parser
in LLDB. Vedant's coverage report [1] shows that it was mostly untested.
Furthermore, the LLVM implementation has a much nicer API and using it
means one less thing to maintain for LLDB.

[1] http://lab.llvm.org:8080/coverage/coverage-reports/index.html

Differential revision: https://reviews.llvm.org/D68305

llvm-svn: 373501