BigFloat rounding no longer works

[dpsanders]

julia> BigFloat(1//10, RoundDown)
1.0000000000000001e-01

julia> BigFloat(1//10, RoundUp)
1.0000000000000001e-01

This is causing failures of IntervalArithmetic.jl with latest Julia master and is a regression from 1.0.2.

cc @simonbyrne

[simonbyrne]

Probably due to #29157: I'll look into it