[OpenMP] Add interface for 5.1 scope construct

The new interface only marks begin/end of a scope construct for
corresponding OMPT events, and we can use existing interfaces for
reduction operations.

Differential Revision: https://reviews.llvm.org/D108062

GitOrigin-RevId: 224f51d879b5060b5a4f57b2774f6f49a5e8951b
3 files changed