Fix assertion failure in copy_type

Fix assertion failure in copy_type

PR exp/20630 points out a simple way to cause an assertion failure in
copy_type -- but this was found in the wild a few times as well.

copy_type only works for objfile-owned types, but there isn't a deep
reason for this.  This patch fixes the bug by updating copy_type to
work for any sort of type.

Better would perhaps be to finally implement type GC, but I still
haven't attempted this.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=20630