Implement the mechanism that simplifies the execution policy/iterator
category dispatching and allows to implement customizations for
the parallel algorithms with adding custom tags
Reviewed By: rodgert, MikeDvorskiy
Differential Revision: https://reviews.llvm.org/D104492
It fixes an ambiguity issue in case of a user has a custom policy and
calls a version of exclusive_scan with binary operation.
Differential Revision: https://reviews.llvm.org/D62719