compiler: don't export function type parameter names

compiler: don't export function type parameter names

When hashing a function type the parameter names are ignored.
Therefore, we should not write them into the export data.
Otherwise, minor changes in the order in which we process the
types can cause the export data to change uselessly, leading to
unnecessary rebuilds.

Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/262818