If the user requested a zero-base-shadow sanitizer, infer -pie and -fPIC when appropriate.

Differential Revision: http://llvm-reviews.chandlerc.com/D502

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@179082 91177308-0d34-0410-b5e6-96231b3b80d8