[clang] [clang] Format "array is too large" diagnostics with separators (PR #160260)

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Sirraide wrote:

> Not sure if `295'147'905'179'352'825'841` is much better than `295147905179352825841` but I think it would be nice to have separators in more diagnostics.

Yeah, it definitely helps I’d say; I wonder if we should insert separators by default for large numbers but that might not make sense in every context.

https://github.com/llvm/llvm-project/pull/160260