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

[Timm Baeder via cfe-commits]

tbaederr wrote:

> Hum... should we put that in `EvaluateKnownConstInt` directly? Bitfields are not the only use case

See https://reviews.llvm.org/D155548 - I can't really support any of this with performance numbers (from my testing, I can't even support that we short-circuit `IntegerLiteral`s...), but this just seems like the right thing to do to me.

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