[clang] [Clang][C++26] Implement Pack Indexing (P2662R3). (PR #72644)

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@@ -4193,6 +4194,13 @@ void CXXNameMangler::mangleType(const PackExpansionType *T) {
   mangleType(T->getPattern());
 }
 
+void CXXNameMangler::mangleType(const PackIndexingType *T) {
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cor3ntin wrote:

I did ping Cameron, who will get back to me,
I think we should land this (in 19) even if the abi is not stable - and wait for it to be to set the feature test macro

https://github.com/llvm/llvm-project/pull/72644