[libcxx] Fix vector annotator size increase in `vector::insert(pos, count, value)`

Summary:
The size of the vector is being increased by `__n` during the call to `__move_range` and not by 1.
This fixes a test failure in `containers/sequences/vector/vector.modifiers/insert_iter_size_value.pass.cpp` when using ASAN.

Reviewers: danalbert, kcc, mclow.lists

Reviewed By: mclow.lists

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D6264