[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/src/ryu/d2s.cpp b/src/ryu/d2s.cpp
index e22bfa4..245c2eb 100644
--- a/src/ryu/d2s.cpp
+++ b/src/ryu/d2s.cpp
@@ -39,6 +39,7 @@
 // Avoid formatting to keep the changes with the original code minimal.
 // clang-format off
 
+#include <__assert>
 #include <__config>
 #include <charconv>