Don't create a null reference in ASTUnit::LoadFromCommandLine.

This change is the last in the pack of five commits
(also see r216691, r216694, r216695, and r216696) that reduces the number
of test failures in "check-clang" invocation in UBSan bootstrap
from 2443 down to 5.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@216697 91177308-0d34-0410-b5e6-96231b3b80d8