constexpr basic_string_view() noexcept;
Effects: Constructs an empty basic_string_view.
Postconditions: size_ == 0 and data_ == nullptr.
constexpr basic_string_view(const charT* str);
Requires: [str, str + traits::length(str)) is a valid range.
Effects: Constructs a basic_string_view, with the postconditions in Table [tab:string.view.ctr.2].
Complexity: Ο(traits::length(str)).
constexpr basic_string_view(const charT* str, size_type len);
Requires: [str, str + len) is a valid range.
Effects: Constructs a basic_string_view, with the postconditions in Table [tab:string.view.ctr.3].