A just what supplies funds to that general fund besides tax dollars?
Social Security is how much in debt?
Revenue from the general fund comes from corporate and individual income taxes, and excise taxes.
So, by reducing the payroll tax for Social Security, that portion of the obligation was shifted to the people that pay income taxes. The IRS provides that information, if you want to know how income tax obligations are distributed.
Social Security is how much in debt?
Social Security has no "debt", as it cannot borrow money. It currently has Trust Fund assets of about $2.8 trillion, from which withdrawals have been made for about 5 years -- since payroll tax revenue (and the contribution we discussed above) has been less than benefits paid.
Looking forward 75 years to 2090, Social Security has a $12.1 trillion unfunded liability, meaning that after adding the Trust Fund, payroll tax revenue, and interest for that period, then subtracting benefits expected to be due during that period, there is a $12.1T shortage.
You can find the details on page 75 of the Social Security Trustees Report (page 83 of the PDF), in Table IV.B6.
https://www.ssa.gov/oact/tr/2016/tr2016.pdf
However, Social Security cannot borrow money. So by law, benefits must be reduced to what can be sustained by incoming revenue. Current forecast is about a 20% reduction in 2034, if no changes are made by then.