[llvm] [ADT] Introduce SetVector::reserve (NFC) (PR #209951)

[Fangrui Song via llvm-commits]

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@@ -102,6 +106,14 @@ class SetVector {
   /// Determine the number of elements in the SetVector.
   [[nodiscard]] size_type size() const { return vector_.size(); }
 
+  /// Reserve space in the SetVector if supported by the underlying containers.
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MaskRay wrote:

Are there instantiations without `reserve`?

https://github.com/llvm/llvm-project/pull/209951