[clang] [Clang] Fix assertion when __block is used on global variables in C mode (PR #194856)

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TPPPP72 wrote:

 > I think we should drop the attribute from the AST. 

Okay, so the current solution is to directly remove the nodes from the AST. Is there any additional work that needs to be done?

And I still have some doubts about @JustinStitt 's point of view, because I've observed that attributes like `__thread` don't mark themselves as invalid nodes after an error declaration, so I'm unsure if retaining `setInvalid` is a good practice.

https://github.com/llvm/llvm-project/pull/194856