Fix handling of collation in SQL-language functions.

Ensure that parameter symbols receive collation from the function's
resolved input collation, and fix inlining to behave properly.

BTW, this commit lays about 90% of the infrastructure needed to support
use of argument names in SQL functions.  Parsing of parameters is now
done via the parser-hook infrastructure ... we'd just need to supply
a column-ref hook ...

src/include/executor/functions.h

@@ -17,9 +17,18 @@
 #include "nodes/execnodes.h"
 #include "tcop/dest.h"
 
+/* This struct is known only within executor/functions.c */
+typedef struct SQLFunctionParseInfo *SQLFunctionParseInfoPtr;
 
 extern Datum fmgr_sql(PG_FUNCTION_ARGS);
 
+extern SQLFunctionParseInfoPtr prepare_sql_fn_parse_info(HeapTuple procedureTuple,
+						  Node *call_expr,
+						  Oid inputCollation);
+
+extern void sql_fn_parser_setup(struct ParseState *pstate,
+								SQLFunctionParseInfoPtr pinfo);
+
 extern bool check_sql_fn_retval(Oid func_id, Oid rettype,
 					List *queryTreeList,
 					bool *modifyTargetList,