Improve the handling of result type coercions in SQL functions.

Use the parser's standard type coercion machinery to convert the
output column(s) of a SQL function's final SELECT or RETURNING
to the type(s) they should have according to the function's declared
result type.  We'll allow any case where an assignment-level
coercion is available.  Previously, we failed unless the required
coercion was a binary-compatible one (and the documentation ignored
this, falsely claiming that the types must match exactly).

Notably, the coercion now accounts for typmods, so that cases where
a SQL function is declared to return a composite type whose columns
are typmod-constrained now behave as one would expect.  Arguably
this aspect is a bug fix, but the overall behavioral change here
seems too large to consider back-patching.

A nice side-effect is that functions can now be inlined in a
few cases where we previously failed to do so because of type
mismatches.

Discussion: https://postgr.es/m/18929.1574895430@sss.pgh.pa.us

src/include/executor/functions.h

@@ -31,10 +31,10 @@ extern void sql_fn_parser_setup(struct ParseState *pstate,
 
 extern void check_sql_fn_statements(List *queryTreeList);
 
-extern bool check_sql_fn_retval(Oid func_id, Oid rettype,
-								List *queryTreeList,
-								bool *modifyTargetList,
-								JunkFilter **junkFilter);
+extern bool check_sql_fn_retval(List *queryTreeList,
+								Oid rettype, TupleDesc rettupdesc,
+								bool insertDroppedCols,
+								List **resultTargetList);
 
 extern DestReceiver *CreateSQLFunctionDestReceiver(void);