[clang] [llvm] [LoopUnroll] Add flag to enforce loop unroll pragma regardless of expensive trip count (PR #180961)

[Shilei Tian via cfe-commits]

shiltian wrote:

> Yes, I know that this is not how it works right now, but I believe this is just an implementation bug, not an intentional choice. The unroll pragmas exist specifically to override these kinds of heuristics, and we should respect them.

+1. We need to fix the root cause instead of symptoms. To me, the `unroll` pragma is essentially an enforcement.

https://github.com/llvm/llvm-project/pull/180961