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

[Jie Fu 傅杰 via cfe-commits]

DamonFool 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.
```

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