[OpenMP] add 4 custom APIs supporting MSVC OMP codegen

This check-in adds 4 APIs to support MSVC, specifically:

* 3 APIs (__kmpc_sections_init, __kmpc_next_section,
   __kmpc_end_sections) to support the dynamic scheduling of OMP sections.
* 1 API (__kmpc_copyprivate_light, a light-weight version of
  __kmpc_copyrprivate) to support the OMP single copyprivate clause.

Differential Revision: https://reviews.llvm.org/D128403