The Federal government.
In order to redeem those bonds, the Feds must issue new debt (or print more money), but that’s just a transfer from one column to the other; it doesn’t raise the national debt.
When your counterparty is in the red to the tune of more than $17 trillion and unfunded future liabilities of several times that amount, you have no “trust fund” and the assets in it are fraudulent. If you want to prioritize payments out of general revenues, fine, but anyone thinking there is some kind of trust fund worth $4 trillion of investments in it probably shouldn’t be handling their own or anyone else’s affairs.