PR51018: Disallow explicit construction of StringRef from SmallString due to ambiguity in C++23

See bug for full details, but basically there's an upcoming ambiguity in
the conversion in `StringRef(SomeSmallString)` - either the implicit
conversion operator (SmallString::operator StringRef) could be used, or
the std::string_view range-based ctor (& then `StringRef(std::string_view)`
would be used)

To address this, make such a conversion invalid up-front - most uses are
more tersely written as `SomeSmallString.str()` anyway, or more clearly
written as `StringRef x = y;` rather than `StringRef x(y);` - so if you
hit this in out-of-tree code, please update in one of those ways.
Hopefully I've fixed everything in tree prior to this patch landing.