[analyzer] Fix scan-build to handle missing output directories.

Cwd::abs_path has a somewhat tricky semantics: if it's operand directory does not exist,
it'll return undefined (see https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=257568).
This may cause scan-build to silently ignore output directory (specified with -o) and
use /tmp instead of trying to create directory. This tiny patch fixes the problem.

A patch by Yury Gribov!

Differential Revision: http://reviews.llvm.org/D14535

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@252797 91177308-0d34-0410-b5e6-96231b3b80d8