[llvm] [ADT] Add `_twine` literal for length-aware `Twine`s (PR #200657)

[Daniil Dudkin via llvm-commits]

unterumarmung wrote:

The main reason I opened the PR is a stylistic one. Creating Twine from string literals using user-defined literals seems natural to me. The avoided `strlen` call is an added bonus. When I have time, I'll try to file this PR plus all LLVM's and Clang's code rewritten using this. I think better results can be achieved than the @aengelke experiment because here no templates are involved, but I'm not sure about this. I think this can be done either after this PR gets merged or before. What do you think?

https://github.com/llvm/llvm-project/pull/200657