[llvm] [DebugInfo] Make DIExpression inherit from Metadata and it always should be unique (PR #79335)

[Adrian Prantl via llvm-commits]

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@@ -2656,31 +2656,27 @@ class DIVariable : public DINode {
 /// DW_OP_stack_value) is the constant variable value.
 ///
 /// TODO: Co-allocate the expression elements.
-/// TODO: Separate from MDNode, or otherwise drop Distinct and Temporary
-/// storage types.
-class DIExpression : public MDNode {
+class DIExpression : public Metadata, ReplaceableMetadataImpl {
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adrian-prantl wrote:

Should we document why DIExpression is not an MDNode here?

https://github.com/llvm/llvm-project/pull/79335