[clang] [Clang] Wide delimiters ('{{{') for expect strings (PR #77326)

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sethp wrote:

@shafik I can, though I'm not quite sure where: my actual use-case is in a file that [doesn't exist yet](https://github.com/llvm/llvm-project/pull/74852/files#diff-f097ac693293d4002c0f9613238b5eaad14b46e30e8c2c9b1f43d8bdedfb06e1R10-R11). There's already accidentally a couple of positive tests for the widened delimiters in equality_tracking.c:, e.g. https://github.com/llvm/llvm-project/blob/ff47989ec238dafe4a68c6a716e8dbccc9f559f5/clang/test/Analysis/equality_tracking.c#L286 

Do those work for you? I didn't spot any way to do negative-case testing ("who verifies the verifier?"), if that's what you had in mind?

https://github.com/llvm/llvm-project/pull/77326