[clang] [Clang] Add thousands-separator to "cannot allocate array; evaluated array bound [...] exceeds the limit" diagnostic (PR #211785)

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VachanVY wrote:

> Please use existing facilities to add thousands-separators, not implement a new one.

@zwuis do you mean `APInt::toString(..., /*InsertSeparators=*/true` (exposed via `llvm::toString` in `llvm/ADT/StringExtras.h`) ?

https://github.com/llvm/llvm-project/pull/211785