[lldb] Silence signed <-> unsigned integer comparison warning

`num_args` is unsigned integer, declared as below:

```
uint32_t num_args = arg_enum->getChildCount();
```

Comparison with the signed `arg_idx` produces a warning when compiled with
-Wsign-compare flag, this patch addresses this simple issue without affecting
any functionality.

Reviewers: davide, asmith
Subscribers: lldb-commits
Differential Revision: https://reviews.llvm.org/D42620

llvm-svn: 323645