Missing return in model_storage_v2.h for non-void methods

[StefanBruens]

Version: main/v9.15

or-tools/ortools/math_opt/storage/model_storage_v2.h

Lines 1128 to 1132 in 79e340f

	template <typename IdType>
	std::vector<VariableId> ModelStorageV2::VariablesInConstraint(
	const IdType) const {
	LOG(FATAL) << "not implementable for ModelStorageV2";
	}

Although LOG(FATAL) aborts, this is not visible for the compiler, and thus compilations fails with -Werror=return-type:

/home/abuild/rpmbuild/BUILD/google-or-tools-9.15-build/or-tools-9.15/ortools/math_opt/storage/model_storage_v2.h:1132:1: error: no return statement in function returning non-void [-Werror=return-type]

The code should be annotated with std::unreachable (C++23) or __builtin_unreachable (GCC/LLVM), __assume(false) (MSVC).