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

[via cfe-commits]

cor3ntin wrote:

> lldb may also need to be fixed?
> 
> ```
> llvm-project/lldb/source/Plugins/TypeSystem/Clang/TypeSystemClang.cpp:4074:11: error: enumeration value 'PackIndexing' not handled in switch [-Werror,-Wswitch]
>   switch (qual_type->getTypeClass()) {
>           ^~~~~~~~~~~~~~~~~~~~~~~~~
> llvm-project/lldb/source/Plugins/TypeSystem/Clang/TypeSystemClang.cpp:4751:11: error: enumeration value 'PackIndexing' not handled in switch [-Werror,-Wswitch]
>   switch (qual_type->getTypeClass()) {
>           ^~~~~~~~~~~~~~~~~~~~~~~~~
> llvm-project/lldb/source/Plugins/TypeSystem/Clang/TypeSystemClang.cpp:5080:11: error: enumeration value 'PackIndexing' not handled in switch [-Werror,-Wswitch]
>   switch (qual_type->getTypeClass()) {
>           ^~~~~~~~~~~~~~~~~~~~~~~~~
> 3 errors generated.
> ```

Yes. (I missed that, thanks). I'm not familiar with lldb codebase but the fix looks trivial.

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