Avoid leaking memory while evaluating arguments for a table function.

ExecMakeTableFunctionResult evaluated the arguments for a function-in-FROM
in the query-lifespan memory context.  This is insignificant in simple
cases where the function relation is scanned only once; but if the function
is in a sub-SELECT or is on the inside of a nested loop, any memory
consumed during argument evaluation can add up quickly.  (The potential for
trouble here had been foreseen long ago, per existing comments; but we'd
not previously seen a complaint from the field about it.)  To fix, create
an additional temporary context just for this purpose.

Per an example from MauMau.  Back-patch to all active branches.

src/include/executor/executor.h

@@ -223,6 +223,7 @@ extern Datum GetAttributeByName(HeapTupleHeader tuple, const char *attname,
 				   bool *isNull);
 extern Tuplestorestate *ExecMakeTableFunctionResult(ExprState *funcexpr,
 							ExprContext *econtext,
+							MemoryContext argContext,
 							TupleDesc expectedDesc,
 							bool randomAccess);
 extern Datum ExecEvalExprSwitchContext(ExprState *expression, ExprContext *econtext,

src/include/nodes/execnodes.h

@@ -1353,6 +1353,7 @@ typedef struct SubqueryScanState
  *		tupdesc				expected return tuple description
  *		tuplestorestate		private state of tuplestore.c
  *		funcexpr			state for function expression being evaluated
+ *		argcontext			memory context to evaluate function arguments in
  * ----------------
  */
 typedef struct FunctionScanState
@@ -1362,6 +1363,7 @@ typedef struct FunctionScanState
 	TupleDesc	tupdesc;
 	Tuplestorestate *tuplestorestate;
 	ExprState  *funcexpr;
+	MemoryContext argcontext;
 } FunctionScanState;
 
 /* ----------------