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

[Benedek Kaibas via llvm-commits]

================
@@ -470,8 +479,8 @@ class Twine {
   /// given SmallVector and a StringRef to the SmallVector's data is returned.
   ///
   /// The returned StringRef's size does not include the null terminator.
-  LLVM_ABI StringRef
-  toNullTerminatedStringRef(SmallVectorImpl<char> &Out) const;
+  LLVM_ABI StringRef toNullTerminatedStringRef(
+      SmallVectorImpl<char> &Out LLVM_LIFETIME_BOUND) const;
----------------
benedekaibas wrote:

> Thinking about this a bit, I think `this` should be annotated for `toNullTerminatedStringRef` , right? That's the whole point of this function that if it's a stringref, it just returns that instead.

Yes, you are right based on the comment:
```text
  /// This returns the twine as a single null terminated 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.
  ///
  /// The returned StringRef's size does not include the null terminator.
```

I think the same also applies to the `toStringRef` function.

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