[perl #117947] Verify lvalueness of XSUBs at run time

If the sub is not visible at compile time, the op tree is flagged such
that pp_entersub will know whether to check the lvalueness of the
called sub.

That check has been in pp_entersub since da1dff9483c.  When I moved
it to pp_entersub in that commit, I only added it to the pure-Perl
branch, not to the XS branch, allowing all XSUBs to be treated as
lvalues if they are not visible at compile time.
(cherry picked from commit 4587c5322c964beac01a38188957ca11026dc766)
perldelta for #117947
[perl #118297] Fix interpolating downgraded variables into upgraded regexp

The code alredy upgraded the pattern if interpolating an upgraded
string into it, but not vice versa.  Just use sv_catsv_nomg() instead
of sv_catpvn_nomg(), so that it can upgrade as necessary.