Move UnpoisonMappedDSO to sanitizer_common.

This is so DFSan will be able to use it.

Differential Revision: http://llvm-reviews.chandlerc.com/D1206

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@187372 91177308-0d34-0410-b5e6-96231b3b80d8