[ThinLTO] Fix thinlto-bad-summary-5.ll in read-only environments (#209513)

The test thinlto-bad-summary-5.ll was failing in read-only environments
(such as sandboxed test runners) because it attempted to write the
output bitcode file to the same directory as the input file, resulting
in a "Permission denied" error.

Fix this by redirecting the output to /dev/null using `-o /dev/null`.
We cannot use `-disable-output` here because the error we want to
test is triggered inside the BitcodeWriter, which is bypassed when
output is disabled.

assisted by gemini

GitOrigin-RevId: d218bc40aba65687ddc2fb616a154b2799631f0f