PR25841: support context vars in functions called from process.mark probes

PR25841: support context vars in functions called from process.mark probes

Extending the prior work to <sys/sdt.h> markers.  This is tricky
because the sdt derived_probes are built with a sequence of
intermediate copies and an ultimate transform to a process.statement
probe.  So the function-cloning & .mark() context-var replacement is
done early, before the synthetic process.statement builder gets a kick
at cloning/etc.  (And the latter doesn't know about sdt $$provider
etc.)

The oddest change is to deep_copy_visitor::visit_functioncall() which
used to clear the copy's referents[] array, but now we preserve (for
reasons elaborated upon in comments).  This has the potential to
impact all the other probes, but initial testing shows nothing
adverse.  No idea why we didn't do this earlier.

Nuked the unused sdt_query::convert_probe() function, not used since
deprecation of sdt-v1.  And an addendum to the message for commit
00ee19ff030f66: removed some ancient symbol-table-only
probe-resolution code machinery.  We can still probe that way, but the
code doesn't actively try to avoid pulling in debuginfo any more.