[clang-tidy] Fix diag message for catch-by-value

Summary:
```
catch (std::exception ex)
{
}
```

Was flagged with "catch handler catches a pointer value".

Reviewers: alexfh, aaron.ballman

Reviewed By: aaron.ballman

Subscribers: cfe-commits, JDevlieghere

Patch by Florian Gross!

Differential Revision: https://reviews.llvm.org/D30592

llvm-svn: 298608