[libcxx] Fix __RAII_IncreaseAnnotator for increases >= 1

Summary: Fix suggested by @mclow.lists on D8109. Store the size of the un-poisoned vector upon construction instead of calculating it later.

Reviewers: titus, mclow.lists, kcc, EricWF

Reviewed By: EricWF

Subscribers: mclow.lists, cfe-commits

Differential Revision: http://reviews.llvm.org/D8172

llvm-svn: 231729