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

[via llvm-commits]

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@@ -254,7 +256,7 @@ class Twine {
   /// We take care here to optimize "" into the empty twine -- this will be
   /// optimized out for string constants. This allows Twine arguments have
   /// default "" values, without introducing unnecessary string constants.
-  /*implicit*/ Twine(const char *Str) {
+  /*implicit*/ Twine(const char *Str LLVM_LIFETIME_BOUND) {
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isuckatcs wrote:

Hmm, this is another tricky case. If the string is empty, it is not lifetime bound, right?

```c++
llvm::Twine(std::string("").c_str());
```

I'm not sure how much this pattern is abused, but it can also result in false positives.

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