pad.c: Fix GH Issue #19463, -DXv fails assert when dumping anonymous constant sub

pad.c: Fix GH Issue #19463, -DXv fails assert when dumping anonymous constant sub

Anonymous constant subs were changed to be implemented internally as
XSUBs in 5.21.6, commit 1567c65ac069266bfe65959430c185babd476538.
This broke DEBUGGING perls running under -DXv which weren't taught
about the new implementation. In ed958fa3156084f3cf4d8c4768716d9e1a11ce91
strict.pm also was changed to use such subs, which then breaks many
uses of -DXv.

See t/run/switchDx.t for an example of code that would trigger this that
does not depend on strict.pm

This fixes the problem and adds a test for -Dx.