llvm-wrapper: fix warning C4244

llvm-wrapper/RustWrapper.cpp(1234): warning C4244: '=': conversion from 'uint64_t' to 'unsigned int', possible loss of data
nice consistency:

uint64_t 6009708b43/llvm/include/llvm/IR/DiagnosticInfo.h (L172)
but unsigned 6009708b43/llvm/include/llvm/IR/DiagnosticInfo.h (L1091)

compiler/rustc_codegen_llvm/src/llvm/ffi.rs

@@ -2254,7 +2254,7 @@ extern "C" {
     pub fn LLVMRustUnpackInlineAsmDiagnostic<'a>(
         DI: &'a DiagnosticInfo,
         level_out: &mut DiagnosticLevel,
-        cookie_out: &mut c_uint,
+        cookie_out: &mut u64,
         message_out: &mut Option<&'a Twine>,
     );