compiler: traverse types of constant expressions

    
    We forgot to ever traverse types of constant expressions.  This rarely
    makes a difference--evidently, since nobody noticed--but it does
    matter when we inline constant expressions: we need to ensure that the
    type is visible to the importing code.
    
    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/194317

From-SVN: r275539