[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/string b/include/string
index 3e7f2e9..892df77 100644
--- a/include/string
+++ b/include/string
@@ -522,6 +522,7 @@
 #include <__algorithm/min.h>
 #include <__algorithm/remove.h>
 #include <__algorithm/remove_if.h>
+#include <__assert>
 #include <__config>
 #include <__debug>
 #include <__ios/fpos.h>