https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82749

Goal

Close the upstream bug. The return types should match the specification in the standard. Ideally, the values should be done in the correct precision.

Acceptance criteria

A list of verification conditions, successful functional tests, or expected outcomes in order to declare this story/task successfully completed.
 * Determine the scope of non-conformance
 * Evaluate ABI impact of changes and if they are feasible
 * The minimal change would be to just convert the internal double values to value_type but that doesn't actually do the calculations in the correct type.
 * Decide what (if anything) to change.

piecewise_linear_distribution<T>::densities() should be of type vector<T> and is vector<double>.