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

[Ran Regev via cfe-commits]

regevran wrote:

Done — tightened the summary per https://llvm.org/docs/DeveloperPolicy.html#commit-messages (dropped the before/after examples and the AI-tools bullet list; the `Assisted-by: AI` trailer covers the transparency requirement).

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