[clang] [llvm] [Clang] C++20 Coroutines: Introduce Frontend Attribute [[clang::coro_await_elidable]] (PR #99282)

[Yuxuan Chen via llvm-commits]

================
@@ -1217,6 +1217,14 @@ def CoroDisableLifetimeBound : InheritableAttr {
   let SimpleHandler = 1;
 }
 
+def CoroAwaitElidable : InheritableAttr {
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yuxuanchen1997 wrote:

I am adding this 
```
- Introduced a new attribute ``[[clang::coro_await_elidable]]`` on coroutine return types
  to express elideability at call sites where the coroutine is co_awaited as a prvalue.
```

wdyt? It's rather succinct but I prefer to leave more details in `AttrDocs.td`.

https://github.com/llvm/llvm-project/pull/99282