Do not store duplicate parents when memoization data is available.

Summary:
Do not store duplicate parents when memoization data is available.
This does not solve the duplication problem, but ameliorates it.

Reviewers: klimek

Subscribers: klimek, cfe-commits

Differential Revision: http://reviews.llvm.org/D4124

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@210902 91177308-0d34-0410-b5e6-96231b3b80d8