[libcxx] Consolidate new/delete replacement in tests and disable it when using sanitizers.

Summary:
MSAN and ASAN also replace new/delete which leads to a link error in these tests. Currently they are unsupported but I think it would be useful if these tests could run with sanitizers.

This patch creates a support header that consolidates the new/delete replacement functionality and checking.
When we are using sanitizers new and delete are no longer replaced and the checks always return true.

Reviewers: mclow.lists, danalbert, jroelofs, EricWF

Reviewed By: EricWF

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D6562