#include_next has unintuitive behavior with clang modules

`#include_next` of a different header behaves differently with clang modules off and on. e.g. `<__mbstate_t.h>` in libc++ tries to `include_next ` to skip its own copy of that header. When modules are off, that successfully gets `` from the C stdlib, but when modules are on it gets the one from libc++.

[include_next_reproducer.zip](https://github.com/user-attachments/files/24228641/include_next_reproducer.zip)

Start with the attached include_next_reproducer.zip and compare resolution of __mbstate_t.h including wchar.h with Clang modules disabled and enabled. Trace how the libc++ and C standard-library headers are selected, then use the reproducer to verify that the intended include_next behavior is consistent or clearly documented.

Written by the indexing model from the issue text.