headers: corecrt_wstring.h: fix libc++ build

libc++ checks _CRT_CONST_CORRECT_OVERLOADS only for MSVC, and it doesn't
check __CORRECT_ISO_CPP_WCHAR_H_PROTO. Instead, it uses
_WCHAR_H_CPLUSPLUS_98_CONFORMANCE_ to decide whether to provide the
correct overloads itself or use the ones provided by the system headers.

Without this macro, libc++ will provide the correct overloads itself,
which will trigger -Wcast-qual because it expects the system-provided
functions (e.g. wcschr) will return wchar_t* instead of const wchar_t*.

Previously, this warning was hidden, because the inclusion chain is:
cwchar -> libc++ wchar.h -> our wchar.h (by #include_next) ->
sec_api/wchar_s.h -> libc++ wchar.h (because no #include next)

Because our wchar.h is treated as a system header (it's placed in a path
that is registered with -internal-isystem), and libc++ wchar.h is
included indirectly by our wchar.h, it's also treated as a system header
by Clang even with -D_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER, and the
warnings are hidden.

After merging wchar_s.h into wchar.h, libc++ wchar.h is no longer
included by our wchar.h, and Clang will treat it as a normal header with
-D_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER, then the warnings occur, and CI
fails because -Werror is enabled.

See also: https://github.com/dragon-archer/mingw-w64/actions/runs/33171856131/job/98856450001#step:6:2011

Signed-off-by: Zhongteng Gui <dragon-archer@outlook.com>
Signed-off-by: LIU Hao <lh_mouse@126.com>