[ubsan] Disable the object size check at -O0

This check currently isn't able to diagnose any issues at -O0, not is it
likely to [1]. Disabling the check at -O0 leads to substantial compile
time and binary size savings.

[1] [cfe-dev] Disabling ubsan's object size check at -O0

Differential Revision: https://reviews.llvm.org/D34563

llvm-svn: 306181