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

This came up during a review: https://github.com/llvm/llvm-project/pull/169010#discussion_r2555372077

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

```c++
""s;
L""s;

""sv;
L""sv;
```

The issue targets Clang and the unused results of the four shown string and string_view literal expressions. First confirm how Clang currently treats these expressions and identify the relevant diagnostic and test locations; done means Clang consistently warns for the intended cases without changing unrelated unused-expression behavior.

Written by the indexing model from the issue text.