Make PL_origfilename a pointer to const char

Commit dd37466 both made `scriptname`
a `const char *` and added the cast to the string literal.  The latter
is unnecessary given the former, and by making PL_origfilename a
`const char *`, we can avoid casting the const away from `scriptname`
as well.

intrpvar.h

@@ -467,7 +467,7 @@ PERLVAR(I, origargv,	char **)
 PERLVAR(I, envgv,	GV *)
 PERLVAR(I, incgv,	GV *)
 PERLVAR(I, hintgv,	GV *)
-PERLVAR(I, origfilename, char *)
+PERLVAR(I, origfilename, const char *)
 PERLVARI(I, xsubfilename, const char *, NULL)
 PERLVAR(I, diehook,	SV *)
 PERLVAR(I, warnhook,	SV *)

perl.c

@@ -4017,15 +4017,15 @@ S_open_script(pTHX_ const char *scriptname, bool dosearch, bool *suidscript)
                 }
                 scriptname = savepv(s + 1);
                 Safefree(PL_origfilename);
-                PL_origfilename = (char *)scriptname;
+                PL_origfilename = scriptname;
             }
         }
     }
 
     CopFILE_free(PL_curcop);
     CopFILE_set(PL_curcop, PL_origfilename);
     if (*PL_origfilename == '-' && PL_origfilename[1] == '\0')
-        scriptname = (char *)"";
+	scriptname = "";
     if (fdscript >= 0) {
         rsfp = PerlIO_fdopen(fdscript,PERL_SCRIPT_MODE);
     }

pod/perldelta.pod

@@ -348,7 +348,7 @@ well.
 
 =item *
 
-XXX
+C<PL_origifilename> is now declared as C<const char *>.
 
 =back