[libc++] Disables -Wweak-vtables diagnostics. (#85577)

This is a preparation to use Clang HEAD in the CI.

GitOrigin-RevId: a77d3d9a2e5decc814119dc4e0a7b4625a6f6490
diff --git a/include/__expected/bad_expected_access.h b/include/__expected/bad_expected_access.h
index 27f01d9..585b4ec 100644
--- a/include/__expected/bad_expected_access.h
+++ b/include/__expected/bad_expected_access.h
@@ -27,6 +27,8 @@
 template <class _Err>
 class bad_expected_access;
 
+_LIBCPP_DIAGNOSTIC_PUSH
+_LIBCPP_CLANG_DIAGNOSTIC_IGNORED("-Wweak-vtables")
 template <>
 class bad_expected_access<void> : public exception {
 protected:
@@ -44,6 +46,7 @@
   // it adds deployment target restrictions.
   _LIBCPP_HIDE_FROM_ABI_VIRTUAL const char* what() const noexcept override { return "bad access to std::expected"; }
 };
+_LIBCPP_DIAGNOSTIC_POP
 
 template <class _Err>
 class bad_expected_access : public bad_expected_access<void> {