Makes scan-build successfully accept non-existing output directories provided via "-o" option. The directory is created in this case. This behavior is conforming to the old perl scan-build implementation.
(http://reviews.llvm.org/D17091)

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@261480 91177308-0d34-0410-b5e6-96231b3b80d8