[validation] explicit Success/Failure ctors for MempoolAcceptResult

Summary:
Makes code more clear and prevents accidentally calling the wrong ctor.

This is a backport of core#20833 [2/13]
https://github.com/bitcoin/bitcoin/pull/20833/commits/578148ded62828a9820398165c41670f4dbb523d

Depends on D12101

Commit backported out of order (standalone refactorings first, new feature last)

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12102