[clang] [LifetimeSafety] Introduce a liveness-based lifetime policy (PR #159991)

[Jan Voung via cfe-commits]

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@@ -974,62 +976,46 @@ static llvm::ImmutableSet<T> join(llvm::ImmutableSet<T> A,
   return A;
 }
 
-/// Checks if set A is a subset of set B.
-template <typename T>
-static bool isSubsetOf(const llvm::ImmutableSet<T> &A,
-                       const llvm::ImmutableSet<T> &B) {
-  // Empty set is a subset of all sets.
-  if (A.isEmpty())
-    return true;
-
-  for (const T &Elem : A)
-    if (!B.contains(Elem))
-      return false;
-  return true;
-}
-
 /// Computes the key-wise union of two ImmutableMaps.
+/// \param SymmetricJoin If true, the join is symmetric, applying JoinValues to
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jvoung wrote:

small point -- it was initially unclear when you would want to do the SymmetricJoin, but after reading more of the PR, it appears to be a performance win

Or, perhaps more relevant to the documentation is when it would be correct to use SymmetricJoin?

https://github.com/llvm/llvm-project/pull/159991