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

[Erich Keane via cfe-commits]

https://github.com/erichkeane commented:

A LOT of stuff here... probably going to need a few more cycles on this to completely grok it.  I didn't spend much time on tests, but would love if we could come up with some 'common patterns' (particularly any that others have sent you as a 'look at this cool thing i can do with this!) to put in tests.

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