Add maybe-unused attribute to a variable.

Summary:
Mark a variable as maybe-unused to prevent a -Wunused-but-set-variable
warning in optimized builds where asserts are removed.Test/first commit
to check setup and understand patch submission process.

Reviewers: srhines, pirama, dblaikie

Reviewed By: dblaikie

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D49756

llvm-svn: 338654