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

[via cfe-commits]

Sirraide wrote:

> I feel like most big numbers in diagnostics won’t be _that_ big

And if we end up having to deal w/ _really_ large numbers, we run into... [other problems](#71675)

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