DataFlowSanitizer: Implement trampolines for function pointers passed to custom functions.

Differential Revision: http://llvm-reviews.chandlerc.com/D1503

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@189408 91177308-0d34-0410-b5e6-96231b3b80d8