[clang] Reapply "[clang] Introduce [[clang::lifetime_capture_by(X)]] (PR #115823)

[Aaron Puchert via cfe-commits]

================
@@ -3918,6 +3918,75 @@ have their lifetimes extended.
   }];
 }
 
+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``.
+
+By default, a reference is considered to refer to its referenced object, a
+pointer is considered to refer to its pointee, a ``std::initializer_list<T>``
+is considered to refer to its underlying array, and aggregates (arrays and
+simple ``struct``\s) are considered to refer to all objects that their
+transitive subobjects refer to.
+
+The capturing entity ``X`` can be one of the following:
+- Another (named) function parameter. 
+  
+  .. code-block:: c++
+
+    void addToSet(std::string_view a [[clang::lifetime_capture_by(s)]], std::set<std::string_view>& s) {
+      s.insert(a);
+    }
+
+- ``this`` (in case of member functions).
+  
+  .. code-block:: c++
+
+    class S {
+      void addToSet(std::string_view a [[clang::lifetime_capture_by(this)]]) {
+        s.insert(a);
+      }
+      std::set<std::string_view> s;
+    };
+
+- 'global', 'unknown' (without quotes).
+  
+  .. code-block:: c++
+
+    std::set<std::string_view> s;
+    void addToSet(std::string_view a [[clang::lifetime_capture_by(global)]]) {
+      s.insert(a);
+    }
+    void addSomewhere(std::string_view a [[clang::lifetime_capture_by(unknown)]]);
+
+The attribute can be applied to the implicit ``this`` parameter of a member
+function by writing the attribute after the function type:
+
+.. code-block:: c++
+
+  struct S {
+    const char *data(std::set<S*>& s) [[clang::lifetime_capture_by(s)]] {
+      s.insert(this);
+    }
+  };
+
+The attribute supports specifying more than one capturing entities:
+
+.. code-block:: c++
+  
+  void addToSets(std::string_view a [[clang::lifetime_capture_by(s1, s2)]],
+                 std::set<std::string_view>& s1,
+                 std::set<std::string_view>& s2) {
+    s1.insert(a);
+    s2.insert(a);
+  }
+
+.. _`lifetimebound`: https://clang.llvm.org/docs/AttributeReference.html#lifetimebound
----------------
aaronpuchert wrote:

There is no need for an external link within the same page. In fact, this breaks the existing `lifetimebound` anchor. The fix is to simply remove this line. (#142967)

https://github.com/llvm/llvm-project/pull/115823