Change MemoryContextMemAllocated to return Size

Commit f2369bc6 switched most of the memory accounting from int64 to
Size, but it forgot to change the MemoryContextMemAllocated return type.
So this fixes that omission.

Discussion: https://www.postgresql.org/message-id/11238.1570200198%40sss.pgh.pa.us

src/backend/utils/mmgr/mcxt.c

@@ -466,10 +466,10 @@ MemoryContextIsEmpty(MemoryContext context)
  * Find the memory allocated to blocks for this memory context. If recurse is
  * true, also include children.
  */
-int64
+Size
 MemoryContextMemAllocated(MemoryContext context, bool recurse)
 {
-	int64 total = context->mem_allocated;
+	Size	total = context->mem_allocated;
 
 	AssertArg(MemoryContextIsValid(context));
 

src/include/utils/memutils.h

@@ -82,7 +82,7 @@ extern void MemoryContextSetParent(MemoryContext context,
 extern Size GetMemoryChunkSpace(void *pointer);
 extern MemoryContext MemoryContextGetParent(MemoryContext context);
 extern bool MemoryContextIsEmpty(MemoryContext context);
-extern int64 MemoryContextMemAllocated(MemoryContext context, bool recurse);
+extern Size MemoryContextMemAllocated(MemoryContext context, bool recurse);
 extern void MemoryContextStats(MemoryContext context);
 extern void MemoryContextStatsDetail(MemoryContext context, int max_children);
 extern void MemoryContextAllowInCriticalSection(MemoryContext context,