C++: add unicode identifier name support

[popkc]

Ctags currently does not support Unicode identifiers in c++, but I found that it only requires modifying one line of code to make it support them.

[masatake]

The C++ related test cases failed on the may platforms.
If you use autotools to build ctags, the failures may be reproduced with following command line:

$ ./configure --enable-debugging
$ make units LANGUAGES=C++

See https://docs.ctags.io/en/latest/testing-parser.html#units-test-facility about the test harness used in ctags.