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

[Jason Rice via cfe-commits]

ricejasonf wrote:

> I wonder if we should rename flat_bindings() to bindings() and bindings() to bindings_as_written() (or similar), as I expect that to be the function we ~always want to use.

I do not have a strong preference, but I kept `bindings()` the same since it represents the structure of the AST and that is how it is used in most places. (like 2x time more than `flat_bindings()`)

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