[llvm-objdump] Disable leak checking on an llvm-objdump test

Summary:
This leak doesn't reproduce locally on macOS 10.12, but is causing
buildbot failures. Disable leak checking until it can be fixed.

Reviewers: sqlbyme, qcolombet, enderby, bruno

Reviewed By: bruno

Subscribers: bruno, llvm-commits

Differential Revision: https://reviews.llvm.org/D38699

llvm-svn: 315337