Constructing a string_view from string literal uses a call to strlen

[rmn30]

The following code will emit a call to strlen:

std::string_view a {"hello"};

This is a bit surprising because it could be evaluated at compile time. It's slightly annoying because it might require adding the 'strings' library on CHERIoT and obviously is slightly suboptimal from a performance point of view. Using a string_view literal doesn't require this call e.g.

using namespace std::literals;
std::string_view a {"hello"sv};

This is by no means urgent and may be an upstream issue but thought it worth noting.