lldb-mi: -var-update can hang when traversing complex types with pointers

Summary:
-var-update calls CMICmdCmdVarUpdate::ExamineSBValueForChange to check if a varObj has been updated. It checks that the varObj is updated, then recurses on all of its children. If a child is a pointer pointing back to a parent node, this will result in an infinite loop, and lldb-mi hanging.

The problem is exposed by packages/Python/lldbsuite/test/tools/lldb-mi/variable/TestMiVar.py, but this test is skipped everywhere.

This patch changes ExamineSBValueForChange to not traverse children of varObjs that are pointers.

Reviewers: ki.stfu, zturner, clayborg, abidh

Reviewed By: clayborg

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D37154

llvm-svn: 312270