[clang] [Clang] Fix assertion when __block is used on global variables in C mode (PR #183988)

[Oliver Hunt via cfe-commits]

ojhunt wrote:

> clang now also rejects `__block` on ObjC instance methods. Neither test nor commit message mention this -- is that an intentional change?
> 
> This breaks some of our builds as some dependencies we have apparently do this (https://source.chromium.org/chromium/chromium/src/+/main:ios/third_party/earl_grey2/src/AppFramework/Synchronization/GREYDispatchQueueTracker.m;l=201?q=GREYDispatchQueueTracker.m)
> 
> Would it be possible to put this in a default-error-mapped warning for a bit, to make it possible to turn this off while fixing code?

That is not intentional. Let me work out why this is happening. Independently it's depressing this didn't trip up existing tests :-/

https://github.com/llvm/llvm-project/pull/183988