[clang] [NFC][clang][AST] Add compile-time dispatch for random access iterators in append() (PR #162000)

[Yanzuo Liu via cfe-commits]

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@@ -192,9 +204,11 @@ class ASTVector {
       this->grow(C, this->size()+NumInputs);
 
     // Copy the new elements over.
-    // TODO: NEED To compile time dispatch on whether in_iter is a random access
-    // iterator to use the fast uninitialized_copy.
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zwuis wrote:

Is the comment correct? Can you confirm that `std::distance` and `std::uninitialized_copy` do NOT perform compile time dispatch? How is `std::uninitialized_copy_n` faster here?

https://github.com/llvm/llvm-project/pull/162000