Add more initializers to quiet a clang warning

Summary:
`struct crashreporter_annotations_t` gained one more `uint64_t` field in
`CRASHREPORTER_ANNOTATIONS_VERSION` 5

causing an annoying clang warning:

```
llvm/lib/Support/PrettyStackTrace.cpp:92:65: warning: missing field 'abort_cause' initializer [-Wmissing-field-initializers]
        = { CRASHREPORTER_ANNOTATIONS_VERSION, 0, 0, 0, 0, 0, 0 };
                                                                ^
1 warning generated
```

Let's fix it.

Patch by Roman Tereshin

Reviewers: qcolombet, echristo, beanz, dexonsmith

Reviewed By: echristo

Subscribers: dsanders, dexonsmith, beanz, echristo, qcolombet, llvm-commits

Differential Revision: https://reviews.llvm.org/D42268

llvm-svn: 323777