[clang] Clang should warn for ignored result of string literals operators

[H-G-Hristov]

This came up during a review: #169010 (comment)

Maybe Clang should warn when string and string_view literals operator result is unused:

  ""s;
  L""s;

  ""sv;
  L""sv;

[philnik777]

More generally, Clang may want to warn if any UDL isn't used.

[AaronBallman]

More generally, Clang may want to warn if any UDL isn't used.

That seems like a reasonable idea; we already warn on things like 12u;: https://godbolt.org/z/YEjY1rMM1