[llvm] [LLVM][ADT] Add support for `std::string += Twine` operator (PR #140644)

[A. Jiang via llvm-commits]

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@@ -578,6 +579,8 @@ namespace llvm {
     return OS;
   }
 
+  std::string &operator+=(std::string &LHS, const Twine &RHS);
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frederick-vs-ja wrote:

Do we want this to be a hidden friend?

https://github.com/llvm/llvm-project/pull/140644