[libc++] cuchar redeclares ::mbstate_t when it's in its own clang module

When cuchar is compiled independently on platforms that don't have <uchar.h>, it doesn't get a declaration of mbstate_t, and so makes an empty declaration for ::mbstate_t. That conflicts with the declarations in __mbstate_t.h and cwchar since both of those headers do get mbstate_t before making ::mbstate_t.

Change `__mbstate_t.h` to just get the underlying declaration for mbstate_t and not make a declaration for ::mbstate_t. Include __mbstate_t.h in uchar.h and wchar.h when their next headers aren't available so that at least mbstate_t gets defined.

Add __std_mbstate_t.h to declare ::mbstate_t for headers that need that when cuchar or cwchar aren't available (because either _LIBCPP_HAS_NO_WIDE_CHARACTERS or _LIBCPP_CXX03_LANG).

Reviewed By: ldionne, Mordante, #libc, philnik

Differential Revision: https://reviews.llvm.org/D148542