Refactor deque to centralize handling of spare blocks.

I have upcoming changes that modify how deque handles spare blocks.
This cleanup is intended to make those changes easier to review
and understand. This patch should have NFC.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@367631 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed