Summary:
Resolving a FIXME in MIVariable, make MIVariable use per session map.
This also helps lldb plugin, which has to enumerate the map to update variables,
otherwise it would also update variables for other sessions.

Test Plan: gdb unit tests and lldb unit tests (on peifeng/lldb-plugin branch)

Reviewers: #kdevelop, apol

Reviewed By: #kdevelop, apol

Subscribers: kdevelop-devel

Differential Revision: https://phabricator.kde.org/D2132