Improve robustness of dependency-generation-crash.c test.

The test wants to provoke a failure when opening the output file.
Using chmod 0 on the output file does not work reliably on all
filesystems or when running the test as root.
Change the test to use a nonexistant directory instead.

Differential Revision: http://reviews.llvm.org/D7620

llvm-svn: 231009