[llvm] [Support][FileCheck] C++26 fixes for P2591 (PR #223284)

[James Henderson via llvm-commits]

https://github.com/jh7370 commented:

I'm not convinced by this, even if it is to fix a C++26 build issue. The fixed code is significantly less readable (in my opinion) than the original. It seems to me like the fix should be different. Would additional explicit `operator+` overloads resolve the problem, so that we don't need any implicit conversion? I.e. something like `operator+(const std::string& Lhs, llvm::StringRef Rhs)` and its invert and/or some involving `Twine` if needed?

https://github.com/llvm/llvm-project/pull/223284