[clang] [clang] Support DecompositionDecl in -ast-print (PR #221711)

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman commented:

I think the array initialization has a bug; if that's a real pain to address as part of this PR, I think this is still fine as-is because it's forward progress in a lot of ways. But if it can be addressed within this PR, that would be nice. WDYT?

https://github.com/llvm/llvm-project/pull/221711