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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> Well, this is not going to make a noticeable difference in runtime. https://reviews.llvm.org/D155548 didn't land because there are no measurements to make where this makes a measurable difference.

Those changes didn't land because no measurements were attempted. Putting up a branch at https://llvm-compile-time-tracker.com/ would help get those measurements to at least start to see if there's benefit or harm from the changes.

> As for my earlier comment, it would also make sense to rename that function to `computeBitWidth()` or just cache the computed value (we compute it when parsing anyway to diagnose 0 size, etc. right?).

Caching the computed value would make sense, but that's sort of the goal of D155548, right? That's a generalized caching mechanism that should mean when we compute it to diagnose 0 size, we never need to re-compute it again.

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