[clang][test] Avoid writing to a potentially write-protected dir (#88258)

This test just checks for the stdout/stderr of clang, but it
incidentally tries to write to `a.out` in the current directory, which
may be write protected. Typically one would write `clang -o %t.o` for a
writeable dir, but since we only care about stdout/stderr, throw away
the object file and just write to /dev/null instead.