Explicitly state the output file.

Summary:
Even though this test is a check for failure, lld still attempts
to open the final output file, which fails when the default "a.out"
file is used and the current directory is read-only. Specifying an
output file works around this problem.

Reviewers: espindola

Subscribers: emaste, MaskRay, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D74523