[llvm] [llvm] Fix string and Twine concatenation for C++26 P2591R5 (PR #227959)

[James Henderson via llvm-commits]

jh7370 wrote:

I should add that there's no reason not to keep an explicit conversion operator to std::string_view, as I assume it's the implicit nature of the operator that is causing the issue (assuming it is a conversion operator at all - I haven't actually checked).

Aside: formatting comment needs addressing before this lands, even if we land this PR as a temporary measure, with a subsequent view to doing the other approach.

https://github.com/llvm/llvm-project/pull/227959