[NFCI] Always initialize BugReport const fields
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
Differential Revision: https://reviews.llvm.org/D66265