[libc++] Move everything related solely to _LIBCPP_ASSERT to its own file

This is the first step towards disentangling the debug mode and assertions
in libc++. This patch doesn't make any functional change: it simply moves
_LIBCPP_ASSERT-related stuff to its own file so as to make it clear that
libc++ assertions and the debug mode are different things. Future patches
will make it possible to enable assertions without enabling the debug
mode.

Differential Revision: https://reviews.llvm.org/D119769

NOKEYCHECK=True
GitOrigin-RevId: f87aa19be64499308fc18f92d048c5fa2d3064d9
diff --git a/include/fstream b/include/fstream
index d7d8fb1..80bda63 100644
--- a/include/fstream
+++ b/include/fstream
@@ -180,9 +180,9 @@
 */
 
 #include <__algorithm/max.h>
+#include <__assert>
 #include <__availability>
 #include <__config>
-#include <__debug>
 #include <__locale>
 #include <__utility/unreachable.h>
 #include <cstdio>