libqc-opt 2.5.2 -> 2.5.18

Summary: Upgrade to a more recent libqc-opt. We have hacks in external/icu4c for this lib, but 2.5.2 was built gainst icu4c 48, 2.5.15 against 50, and 2.5.18 against 51 (which is current)

Test Plan: verified

Reviewers: randall.rushing, jriordan001, anders3408

Differential Revision: http://review.codefirex.com/D329