[clang] [llvm] [ADT] Make canonicalization a compile-time parameter of ImmutableSet/Map (PR #209300)

[Utkarsh Saxena via cfe-commits]

================
@@ -34,15 +34,23 @@ template <typename Tag> struct ID {
   }
 };
 
-/// Computes the union of two ImmutableSets.
+/// The lifetime analyses do not benefit from canonicalizing their immutable
+/// collections, so they opt out of it via these aliases.
 template <typename T>
-llvm::ImmutableSet<T> join(llvm::ImmutableSet<T> A, llvm::ImmutableSet<T> B,
-                           typename llvm::ImmutableSet<T>::Factory &F) {
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usx95 wrote:

I think now we can restore the previous more-special template.
```cpp
template <typename T>
SetTy <T> join(SetTy<T> A, SetTy<T> B, typename SetTy<T>::Factory &F) {
```

Same for map join.

https://github.com/llvm/llvm-project/pull/209300