Simplify use of AllocSetContextCreate() wrapper macro.

We can allow this macro to accept either abbreviated or non-abbreviated
allocation parameters by making use of __VA_ARGS__.  As noted by Andres
Freund, it's unlikely that any compiler would have __builtin_constant_p
but not __VA_ARGS__, so this gives up little or no error checking, and
it avoids a minor but annoying API break for extensions.

With this change, there is no reason for anybody to call
AllocSetContextCreateExtended directly, so in HEAD I renamed it to
AllocSetContextCreateInternal.  It's probably too late for an ABI
break like that in 11, though.

Discussion: https://postgr.es/m/20181012170355.bhxi273skjt6sag4@alap3.anarazel.de

src/backend/utils/mmgr/aset.c

@@ -371,7 +371,7 @@ AllocSetFreeIndex(Size size)
 
 
 /*
- * AllocSetContextCreateExtended
+ * AllocSetContextCreateInternal
  *		Create a new AllocSet context.
  *
  * parent: parent context, or NULL if top-level context
@@ -381,11 +381,13 @@ AllocSetFreeIndex(Size size)
  * maxBlockSize: maximum allocation block size
  *
  * Most callers should abstract the context size parameters using a macro
- * such as ALLOCSET_DEFAULT_SIZES.  (This is now *required* when going
- * through the AllocSetContextCreate macro.)
+ * such as ALLOCSET_DEFAULT_SIZES.
+ *
+ * Note: don't call this directly; go through the wrapper macro
+ * AllocSetContextCreate.
  */
 MemoryContext
-AllocSetContextCreateExtended(MemoryContext parent,
+AllocSetContextCreateInternal(MemoryContext parent,
 							  const char *name,
 							  Size minContextSize,
 							  Size initBlockSize,