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

[Nico Weber via cfe-commits]

nico 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?

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