fixup to "avoid identical stack traces" - try 2

fixup to "avoid identical stack traces" - try 2

GH #15109, #17567

[ this commit was originally applied as v5.31.9-121-gfb8188b84d, but was
quickly reverted by v5.31.9-124-g6311900a66. I'm now -re-applying it,
but with a 'SAVEFREEOP(PL_curcop)' added, which was missing from the
original commit. ]

My original fix for this issue, v5.31.6-141-gf2f32cd638
made a shallow copy of &PL_compiling. However, for non-default
warning bits, this made two COPs share the malloced() cop_warnings,
and bad things ensured. In particular this was flagged up in:

    GH #17567: "BBC: AYOUNG/OpenVZ-0.01.tar.gz"

The fix in this commit is to do a deep copy of the COP using
newSTATEOP().