[clang] [clang][AST] Invalidate DecompositionDecl if it has invalid initializer. (PR #72428)

[Henrik G. Olsson via cfe-commits]

hnrklssn wrote:

> > What differentiates the DecompositionDecl such that we need to mark the decl invalid when there's an error in the RHS, while for other decls we don't? It seems inconsistent.
> 
> 
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> DecompositionDecl (aka structure binding) is special here, a legal DecompositionDecl must be declared with a deduced `auto` type, so the its initializer should play part of the decl's invalid bit. For the error case where a DecompositionDecl with a non-deduced type, clang still builds the Decl AST node (with invalid bit) for better recovery. There are some oversight cases. This patch is fixing those.

Ah that makes sense then. I didn't realise it had to be `auto`.

https://github.com/llvm/llvm-project/pull/72428