[libc++] Add missing short wchar handling for codecvt_utf8, codecvt_utf16 and codecvt_utf8_utf16

Summary:
AIX have 2 byte wchar in 32 bit mode and 4 byte wchar in 64 bit mode.
This patch add more missing short wchar handling under the existing _LIBCPP_SHORT_WCHAR macro.

Marked test case ctor_move.pass.cpp as XFAIL for 32-bit mode on AIX because UTF-8 constants used cannot be converted to 2-byte wchar (by xingxue).

Authored by: jasonliu

Reviewed by: ldionne, zibi, SeanP, libc++

Differential Revision: https://reviews.llvm.org/D100777