[libc++] `std.functional.hash` should be exported from Clang modules for some standard library headers

[frederick-vs-ja]

Per [unord.hash]/2, components in <__functional/hash.h> should be available in the following headers:

• <bitset>
• <chrono> (since C++26; changes being implemented)
• <coroutine>
• <filesystem>
• <functional>
• <memory>
• <optional>
• <stacktrace> (not yet implemented)
• <string>
• <string_view>
• <system_error>
• <text_encoding> (not yet implemented)
• <typeindex> (buggy now; see Using std::hash through <typeindex> does not compile for builtin arithmetic types #129723)
• <thread>
• <variant>
• <vector>
• <utility> (since C++26)

In Clang module builds, it should also be valid to obtain the components by including one of the these headers. However, it was observed in #169611 that this didn't work.

Presumably we need to either

• fix module.modulemap.in by exporting std.functional.hash wherever suitable, or
• directly include <__functional/hash.h> in these standard library headers.