[clang] [clang] Allow 'convergent' to be a statement attribute (PR #100637)

[via cfe-commits]

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+// RUN: %clang_cc1 -emit-llvm %s -triple x86_64-unknown-linux-gnu -o - | FileCheck %s
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darkbuck wrote:

> Consider using update_cc_test_checks.py.
> 
> I'd like to see checks that we emit convergence tokens on targets where that's the default.

However, it seems that the script doesn't generate checks on attribute definitions. I tried '--check-attributes`, but it only added function attributes.

https://github.com/llvm/llvm-project/pull/100637