Fix our `llvm::Bool` typedef to be signed, to match `LLVMBool`
In the LLVM-C API, boolean values are passed as `typedef int LLVMBool`, but our Rust-side typedef was using `c_uint` instead.
Signed and unsigned integers have the same ABI on most platforms, but that isn't universally true, so we should prefer to be consistent with LLVM.
https://github.com/rust-lang/llvm-project/blob/1268e87/llvm/include/llvm-c/Types.h#L28
6d79df6867