[OpenMP] libomp minor cleanup

Cleanup changes:
- check value read from file;
- remove dead code;
- make unsigned variable to read hexadecimal number to;
- add debug assertion to check ref count.

Differential Revision: https://reviews.llvm.org/D96893

GitOrigin-RevId: 1df6e58e55a33991054f4041d87200bf1b217197
3 files changed