Provide query source text when parsing a SQL-standard function body.

Without this, we lose error cursor positions, as shown in the
modified regression test result.

Discussion: https://postgr.es/m/2197698.1617984583@sss.pgh.pa.us

src/backend/commands/functioncmds.c

@@ -852,7 +852,9 @@ static void
 interpret_AS_clause(Oid languageOid, const char *languageName,
 					char *funcname, List *as, Node *sql_body_in,
 					List *parameterTypes, List *inParameterNames,
-					char **prosrc_str_p, char **probin_str_p, Node **sql_body_out)
+					char **prosrc_str_p, char **probin_str_p,
+					Node **sql_body_out,
+					const char *queryString)
 {
 	if (!sql_body_in && !as)
 		ereport(ERROR,
@@ -929,6 +931,7 @@ interpret_AS_clause(Oid languageOid, const char *languageName,
 				Query	   *q;
 				ParseState *pstate = make_parsestate(NULL);
 
+				pstate->p_sourcetext = queryString;
 				sql_fn_parser_setup(pstate, pinfo);
 				q = transformStmt(pstate, stmt);
 				if (q->commandType == CMD_UTILITY)
@@ -947,6 +950,7 @@ interpret_AS_clause(Oid languageOid, const char *languageName,
 			Query	   *q;
 			ParseState *pstate = make_parsestate(NULL);
 
+			pstate->p_sourcetext = queryString;
 			sql_fn_parser_setup(pstate, pinfo);
 			q = transformStmt(pstate, sql_body_in);
 			if (q->commandType == CMD_UTILITY)
@@ -954,6 +958,7 @@ interpret_AS_clause(Oid languageOid, const char *languageName,
 						errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
 						errmsg("%s is not yet supported in unquoted SQL function body",
 							   GetCommandTagName(CreateCommandTag(q->utilityStmt))));
+			free_parsestate(pstate);
 
 			*sql_body_out = (Node *) q;
 		}
@@ -1220,7 +1225,8 @@ CreateFunction(ParseState *pstate, CreateFunctionStmt *stmt)
 
 	interpret_AS_clause(languageOid, language, funcname, as_clause, stmt->sql_body,
 						parameterTypes_list, inParameterNames_list,
-						&prosrc_str, &probin_str, &prosqlbody);
+						&prosrc_str, &probin_str, &prosqlbody,
+						pstate->p_sourcetext);
 
 	/*
 	 * Set default values for COST and ROWS depending on other parameters;

src/test/regress/expected/create_function_3.out

@@ -295,6 +295,8 @@ CREATE FUNCTION functest_S_xx(x date) RETURNS boolean
     LANGUAGE SQL
     RETURN x > 1;
 ERROR:  operator does not exist: date > integer
+LINE 3:     RETURN x > 1;
+                     ^
 HINT:  No operator matches the given name and argument types. You might need to add explicit type casts.
 -- tricky parsing
 CREATE FUNCTION functest_S_15(x int) RETURNS boolean