[llvm] [ADT] Add `[[clang::lifetimebound]]` annotations to Twine.h (PR #210474)

[via llvm-commits]

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@@ -458,7 +466,8 @@ class Twine {
   /// This returns the twine as a single StringRef if it can be
   /// represented as such. Otherwise the twine is written into the given
   /// SmallVector and a StringRef to the SmallVector's data is returned.
-  StringRef toStringRef(SmallVectorImpl<char> &Out) const {
+  StringRef toStringRef(SmallVectorImpl<char> &Out LLVM_LIFETIME_BOUND) const
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isuckatcs wrote:

> I think "out" outlives the Twine - thus that annotation is wrong.

For this annotation, doesn't it have to outlive the returned `StringRef`?

```c++
  StringRef toStringRef(SmallVectorImpl<char> &Out) const {
    if (isSingleStringRef())
      return getSingleStringRef();
    toVector(Out);
    return StringRef(Out.data(), Out.size());
  }
```

The returned `StringRef` references the data of `Out`. I think both `Out` and `this` should be annotated.

https://github.com/llvm/llvm-project/pull/210474