[UBSan] Add the ability to dump call stacks to -fsanitize=vptr

This change introduces the first UBSan-specific runtime flag: print_stacktrace
(off by default). It can be set in UBSAN_OPTIONS to unwind and print call stacks
in addition to diagnostic messages. For now these stacks are printed only
in vptr checker.

This change is based on http://reviews.llvm.org/D4410 by Byoungyoung Lee!