Rollup merge of #145817 - Zalathar:llvm-bool, r=workingjubilee

cg_llvm: Replace the `llvm::Bool` typedef with a proper newtype

This should be nicer and more type-safe than the old typedef for `c_int`/`i32`.

Using `#[repr(transparent)]` should ensure that it's still ABI-compatible.

Author: Zalathar