Add a spelling of pass_object_size that uses __builtin_dynamic_object_size
The attribute pass_dynamic_object_size(n) behaves exactly like
pass_object_size(n), but instead of evaluating builtin_object_size on calls,
it evaluates builtin_dynamic_object_size, which has the potential to produce
runtime code when the object size can't be determined statically.
Differential revision: https://reviews.llvm.org/D58757