[clang] [clang] Avoid evaluating the BitWidth expression over and over again (PR #66203)

[via cfe-commits]

cor3ntin wrote:

> Ah, I didn't realize you had measured on your own. But yeah, having some concrete numbers to play with would help.

I'd like to see rough numbers too but I'm really hoping we can progress https://reviews.llvm.org/D155548.
I think it's the worse case it's harmless, at best it could have a pretty big positive impact.

https://github.com/llvm/llvm-project/pull/66203