Summary:
Undefined Behavior Sanitizer (ubsan) is //a good thing// which will help us to find sneaky bugs with low cost. Please see http://developerblog.redhat.com/2014/10/16/gcc-undefined-behavior-sanitizer-ubsan/ for more details and official GCC documentation for more context: https://gcc.gnu.org/onlinedocs/gcc/Instrumentation-Options.html.

Changes itself are quite simple and pretty much imitating whatever is implemented for ASan.

Hooking the UBsan validation build to Sandcastle is a separate step and will be dealt as separate diff because code is in internal repository.

Test Plan: Make sure that that there no regressions when it comes to builds and test pass rate.

Reviewers: leveldb, sdong, IslamAbdelRahman, yhchiang

Reviewed By: yhchiang

Subscribers: andrewkr, dhruba

Differential Revision: https://reviews.facebook.net/D56049