[OpenMP] Fixed the link error that cannot find static data member

Constant static data member can be defined in the class without another
define after the class in C++17. Although it is C++17, Clang can still handle it
even w/o the flag for C++17. Unluckily, GCC cannot handle that.

Reviewed By: jhuber6

Differential Revision: https://reviews.llvm.org/D94541