Fix leak in lib/CodeGen/CGException.cpp, PR18318

Summary: This fixes the leak described in http://llvm.org/bugs/show_bug.cgi?id=18318

Reviewers: chandlerc, dblaikie

Reviewed By: chandlerc

CC: cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D2474

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@198857 91177308-0d34-0410-b5e6-96231b3b80d8