ICU's UChar was uint16_t (non-Win) or wchar_t (Windows). It's switching
to char16_t in both C/C++ API. It needs some changes. Fortunately,
v8 needs only a couple of changes because v8 has been using
reinterpret_cast in many places calling ICU API.

This change was confirmed to work fine with ICU-59-to-be.

BUG=v8:6062
TEST=trybot

Review-Url: https://codereview.chromium.org/2738503008
Cr-Commit-Position: refs/heads/master@{#43707}