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

[Timm Baeder via cfe-commits]

tbaederr wrote:

@cor3ntin Looking at this again... while I'd also like to see the general solution, I'd probably apply this anyway since it makes it clearer that this function can actually be fast. Its name is terrible for what it does. It sounds like it's a simply getter but in reality it might be pretty costly to call. This patch would mitigate this somewhat and make the performance expectations clearer when reading the code of this function.

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