[lldb] Fixed UB in CPlusPlusLanguage plug-in (#158304)
C++11 allows the use of Universal Character Names (UCNs) in identifiers,
including function names.
According to the spec the behavior of std::isalpha(ch) and
std::isalnum(ch) is undefined if the argument's value is neither
representable as unsigned char nor equal to EOF. To use these functions
safely with plain chars (or signed chars), the argument should first be
converted to unsigned char.
