Ignore IOError from missing child.log.

Summary:
TestMiCliSupport (among other TestMi*) fails on Linux with

```
IOError: [Errno 2] No such file or directory: 'child.log'
```

Reviewers: clayborg, ki.stfu

Reviewed By: ki.stfu

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D8843

llvm-svn: 234191