ICU-20904 Don't use char16_t with C++98/03

ICU-20904 Don't use char16_t with C++98/03

When C code includes the ICU headers, the UChar type is defined to be
uint16_t. But when C++ code includes the headers, UChar is char16_t
even when U_SHOW_CPLUSPLUS_API has been set to 0. Apart from arguably
being an inconsistency in the API, this means that C++98 or C++03 code
can't use the C API even though C99 code can.

So, change unicode/umachine.h to check not just whether __cplusplus is
defined but the value of U_CPLUSPLUS_VERSION when deciding how to
typedef UChar.