[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 remove `setInvalidDecl` to maintain consistency here.

https://github.com/llvm/llvm-project/blob/8b9cfeac98b99c3edfe0b5f71c8a11044750e50b/clang/lib/Sema/SemaObjC.cpp#L1701-L1705

https://github.com/llvm/llvm-project/blob/8b9cfeac98b99c3edfe0b5f71c8a11044750e50b/clang/lib/Sema/SemaObjC.cpp#L1709-L1712

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