[clang] Enable AST mutation in the constant evaluator (PR #115168)

[Erich Keane via cfe-commits]

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@@ -352,6 +353,16 @@ struct SkipBodyInfo {
   NamedDecl *New = nullptr;
 };
 
+/// Implementation of EvalASTMutator interface that enables constant evaluator
+/// to modify AST, e.g. to instantiate templates.
+struct SemaASTMutator : EvalASTMutator {
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erichkeane wrote:

I wonder if `Sema` should just be the implementer here, rather than a separate mutator object?  Mark it `final`, and we get free devirtualization in Sema, but that way we can more simply add elements here without the wrapper here.

https://github.com/llvm/llvm-project/pull/115168