[llvm] Add the 'initialized' attribute langref and support (PR #84803)

[Arthur Eubanks via llvm-commits]

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@@ -226,6 +236,10 @@ class Attribute {
   /// attribute to be a ConstantRange attribute.
   ConstantRange getValueAsConstantRange() const;
 
+  /// Return the attribute's value as a const range list. This requires the
+  /// attribute to be a const range list attribute.
+  SmallVector<std::pair<int64_t, int64_t>, 16> getValueAsRanges() const;
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aeubanks wrote:

we should use `ConstantRange` instead of `std::pair`, especially since it already has intersection/union methods

https://github.com/llvm/llvm-project/pull/84803