Improve ValueObject::GetValueDidChange test; Add a comment for it

Summary: This patch adds a few comments for GetValueDidChange and contains improvements for TestValueVarUpdate.py test which checks ValueObject::GetValueDidChange for complex types.

Reviewers: zturner, granata.enrico, clayborg

Reviewed By: clayborg

Subscribers: jingham, lldb-commits, granata.enrico, zturner, clayborg

Differential Revision: http://reviews.llvm.org/D8103

llvm-svn: 231526