[clang] [clang] Format bitfield width diagnostics with thousands-separators (PR #117763)

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cor3ntin wrote:

> Wondering if we should have a toString() version that does this automatically,

Yes, I think either a `toStringWithDigitSeparators` function, or an overload taking an enum rather than a whole bunch of bool would improve this code a lot.

https://github.com/llvm/llvm-project/pull/117763