Ternary expressions of interpolated strings should be able to produce FormattableString

[lcsondes]

This doesn't compile:

var x = FormattableString.Invariant(condition ? $"a" : $"b");
//                                  ~~~~~~~~~~~~~~~~~~~~~~~
// error CS1503: Argument 1: cannot convert from 'string' to 'System.FormattableString'

As a workaround, either side can be explicitly cast to FormattableString, but this shouldn't be necessary.