[analyzer][NFC] Make sure that the BugReport is not modified during the...

[analyzer][NFC] Make sure that the BugReport is not modified during the construction of non-visitor pieces

I feel this is kinda important, because in a followup patch I'm adding different
kinds of interestingness, and propagating the correct kind in BugReporter.cpp is
just one less thing to worry about.

Differential Revision: https://reviews.llvm.org/D65578

llvm-svn: 368755