[NFCI] Always initialize BugReport const fields

Summary:
Some compilers require that const fields of an object must be explicitly
initialized by the constructor. I ran into this issue building with clang
3.8 on Ubuntu 16.04.

Reviewers: compnerd, Szelethus, NoQ

Subscribers: cfe-commits, llvm-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D66265

llvm-svn: 368950