[clang] [llvm] [clang] Implement lifetime analysis for lifetime_capture_by(X) (PR #115921)

[Haojian Wu via cfe-commits]

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@@ -3922,16 +3922,41 @@ def LifetimeCaptureByDocs : Documentation {
   let Category = DocCatFunction;
   let Content = [{
     Similar to `lifetimebound`_, the ``lifetime_capture_by(X)`` attribute on a function
-parameter or implicit object parameter indicates that that objects that are referred to
-by that parameter may also be referred to by the capturing entity ``X``.
+parameter or implicit object parameter indicates that objects that are referred to by
+that parameter may also be referred to by a capturing entity ``X``.
+
+Below is a list of types of the parameters and what they're considered to refer to:
+
+- A reference param is considered to refer to its referenced object.
+- A pointer param is considered to refer to its pointee.
+- A ``std::initializer_list<T>`` is considered to refer to its underlying array.
+- Aggregates (arrays and simple ``struct``\s) are considered to refer to all
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hokein wrote:

Can you add a test for the `array` and `initilizer_list` case?

For the simple `struct` case (e.g. `void capture(MyStruct abc)`), my understanding is that our implementation doesn't support it. 

https://github.com/llvm/llvm-project/pull/115921