Get further through the build with read-only optrees. Free()ing cops

Get further through the build with read-only optrees. Free()ing cops
as currently implemented seems to want write activity to them. Also we
save a field in PL_curcop at one point, and sometimes this can be in
a read only COP. If so, the end of scope restoration is always to the
same value, but it's easier to cheat and skip the restoration than to
dig around and avoid the save.

p4raw-id: //depot/perl@30843