Add a 'dynamic' parameter to the objectsize intrinsic
This is meant to be used with clang's __builtin_dynamic_object_size.
When 'true' is passed to this parameter, the intrinsic has the
potential to be folded into instructions that will be evaluated
at run time. When 'false', the objectsize intrinsic behaviour is
Differential revision: https://reviews.llvm.org/D56761