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


git-svn-id: 4c4cc70b1ef44ba2b7963015e681894188cea27e@323645 91177308-0d34-0410-b5e6-96231b3b80d8

apple/swift-lldb,apple/swift-lldb,llvm-mirror/lldb,apple/swift-lldb,llvm-mirror/lldb,apple/swift-lldb,llvm-mirror/lldb,apple/swift-lldb,llvm-mirror/lldb,apple/swift-lldb,llvm-mirror/lldb