[OpenMP] Expose the state in the header to allow non-lto optimizations

We used to inline the `lookup` calls such that the runtime had "known"
access offsets when it was shipped. With the new static library build it
doesn't as the lookup is an indirection we cannot look through. This
should help us optimize the code better until we can do LTO for the
runtime again.

Differential Revision: https://reviews.llvm.org/D130111