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

[Aaron Ballman via cfe-commits]

AaronBallman 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](https://github.com/llvm/llvm-project/issues/71675)

Yeah, this feels like we're dancing around the same root cause for both issues -- printing really large numbers isn't necessarily helpful. I think this PR is reasonable though; some of the values are getting easier to read, some might be getting harder to read but were already hard to read. I don't think any are getting harder to read that were previously easy to read, but this is pretty subjective stuff too.

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