[clang-query] Continue if compilation command not found for some files

When searching for a code pattern in an entire project with a
compilation database it's tempting to run

```
clang-query **.cpp
```

And yet, that often breaks because some files are just not in the
compilation database: tests, sample code, etc..
clang-query should not stop when encountering such cases.

Differential Revision: https://reviews.llvm.org/D51183

llvm-svn: 348328