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

[Akira Hatanaka via cfe-commits]

ahatanak wrote:

I don't think `setInvalidDecl` is needed in this case. The attribute is rejected before it is added to the declaration and the declaration itself remains valid.

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