[Reproducers] Instrumentation Framework: Serialization

This is the is serialization/deserialization part of the reproducer
instrumentation framework.

For all the details refer to the RFC on the mailing list:
http://lists.llvm.org/pipermail/lldb-dev/2019-January/014530.html

Differential revision: https://reviews.llvm.org/D57714

llvm-svn: 353195