[OpenMP] Introduce a new worksharing RTL function for distribute

This patch adds a new RTL function for worksharing. Currently we use
`__kmpc_for_static_init` for both the `distribute` and `parallel`
portion of the loop clause. This patch replaces the `distribute` portion
with a new runtime call `__kmpc_distribute_static_init`. Currently this
will be used exactly the same way, but will make it easier in the future
to fine-tune the distribute and parallel portion of the loop.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D110429