UBSan now uses prologue data instead of prefix data

As the semantics of prefix data has changed. See D6454.

Patch by Ben Gamari!

Test Plan: Testsuite

Differential Revision: http://reviews.llvm.org/D6489

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@223190 91177308-0d34-0410-b5e6-96231b3b80d8