Use Flags enum in AddArg() (follow-up)

Summary:
Due to an out-of-sequence backport (D5767, D6473), this change was missed in D5767.

Depends on D10304

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10305