Teach pattern_fixed_prefix() about collations.

This is necessary, not optional, now that ILIKE and regexes are collation
aware --- else we might derive a wrong comparison constant for index
optimized pattern matches.

src/include/utils/selfuncs.h

@@ -132,6 +132,7 @@ extern double histogram_selectivity(VariableStatData *vardata, FmgrInfo *opproc,
 
 extern Pattern_Prefix_Status pattern_fixed_prefix(Const *patt,
 					 Pattern_Type ptype,
+					 Oid collation,
 					 Const **prefix,
 					 Const **rest);
 extern Const *make_greater_string(const Const *str_const, FmgrInfo *ltproc);