[Driver] Conform warn_drv_object_size_disabled_O0 to DefaultWarnNoError

This diagnostic triggers when -fsanitize=object-size is explicitly
specified but will be a no-op (i.e, at -O0).

This diagnostic should not fail a -Werror build because it's just an
explanatory note to the user. It's not always actionable.

For example, a user may not be able to simply disable object-size,
because they want it enabled in optimized builds.

rdar://42128447

llvm-svn: 336937