Cache the memoization data for DynTypedNodes.

Summary:
Cache the memoization data for DynTypedNodes.
It is free to do so on create(), but expensive to determine dynamically.

Reviewers: klimek

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D5101

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@216671 91177308-0d34-0410-b5e6-96231b3b80d8