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.

compiler/rustc_codegen_llvm/src/llvm/ffi.rs

@@ -17,7 +17,9 @@ use super::debuginfo::{
     DebugEmissionKind, DebugNameTableKind,
 };
 
-pub type Bool = c_uint;
+/// In the LLVM-C API, boolean values are passed as `typedef int LLVMBool`,
+/// which has a different ABI from Rust or C++ `bool`.
+pub type Bool = c_int;
 
 pub const True: Bool = 1 as Bool;
 pub const False: Bool = 0 as Bool;