By taking the std::string by value in the constructor, this allows for
certain situations where copies can be elided entirely (when moving an
instance into the constructor)
e.g.
std::string var = ...
...
... = LiteralString(std::move(var)) // Or whatever other initialization
// is done.
No copy will be done in this case, the move transfers it into the
constructor, and then the move within the initializer list transfers it
into the member variable.
tl;dr: This allows the calling code to potentially construct less
std::string instances by allowing moving into the parameters themselves.
