[clang] [Clang][P1061] Add stuctured binding packs (PR #121417)

[Erich Keane via cfe-commits]

erichkeane wrote:

As a quick note, the best way to check serialization/deserialization is a PCH /restore test.  I did a bunch in `SemaOpenACC` that you can use as an example (see a bunch ending with `-ast` in the name).  Basically, you just do the right command-line incantation, plus a #ifdef around the whole file.  Write your code that has the correct AST nodes in place, then it does a 'write to a PCH file', then does a 'load from a PCH file'.  You then use `CHECK` lines via `FileCheck` to make sure it is the same before and after the serialization.

https://github.com/llvm/llvm-project/pull/121417