Fix assertion failure with our subs and constants

The code path that looks up ‘our’ subs was not vivifying GVs in
the symbol table, and it was assuming that anything it found
would be a GV.

Constants created by ‘use constant’ are stored as simple scalar refs,
when possible, instead of GVs with CVs.

So GvCV() was called on an RV, and blew up.

$ ./miniperl -Ilib -Mfeature=lexical_subs -e 'our sub foo; BEGIN { delete $::{foo}; } use constant foo=>3; warn foo()'
The lexical_subs feature is experimental at -e line 1.
Assertion failed: (SvTYPE(_gvgp) == SVt_PVGV || SvTYPE(_gvgp) == SVt_PVLV), function Perl_yylex, file toke.c, line 6566.
Abort trap: 6