Pass char16_t* instead of char* to [Qualified]Identifier()

Since https://commits.kde.org/kdevelop/1d9717164cc1eaf6919090ea21053864fce44b2f
these constructors take a QStringView. Passing an ordinary const char[]
string literal created a temporary QString, which entailed memory
allocation and deallocation. Passing a UTF-16 const char16_t[] string
literal creates a QStringView that simply references the string
literal's static storage, which is much more efficient.