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

[Zalathar]

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.

[rustbot]

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[rustbot]

Some changes occurred in compiler/rustc_codegen_llvm/src/builder/autodiff.rs

cc @ZuseZ4

[hkBst]

Looks good, except please clarify the behavior of non-standard values (those greater than 1, and smaller than 0).

View changes since this review

[Zalathar]

Added a comment explaining the handling of boolean values that aren't 0 or 1 (diff).

Note that technically this is a change from the pre-existing behaviour (which tends to compare against True), but it's unlikely that this matters, and if it does matter then the new behaviour seems more likely to be correct.