[JSON][PM] Port json import/export over to new pm

Summary:
I pulled out all functionality into static functions, and use those both
in the legacy passes and in the new ones.

Reviewers: grosser, Meinersbur, bollu

Reviewed By: Meinersbur

Subscribers: llvm-commits, pollydev

Differential Revision: https://reviews.llvm.org/D36578

llvm-svn: 310597