UBSan: crash less often on corrupted Vtables.

Summary:
This CL adds a weak check for a Vtable prefix: for a well-formed
Vtable, we require the prefix to be within [-1<<20; 1<<20].

Practically, this solves most of the known cases when UBSan segfaults
without providing any useful diagnostics.

Reviewers: pcc

Subscribers: kubabrecka

Differential Revision: http://reviews.llvm.org/D19750

llvm-svn: 271560