[Analyzer][StreamChecker] Updated initialization of BugType's.

Summary:
BugType objects are initialized in-class instead of by lazy initialization.
FuchsiaHandleChecker does this already.

Reviewers: Szelethus, baloghadamsoftware, martong

Reviewed By: Szelethus

Subscribers: rnkovacs, xazax.hun, baloghadamsoftware, szepet, a.sidorin, mikhail.ramalho, Szelethus, donat.nagy, dkrupp, gamesh411, Charusso, martong, ASDenysPetrov, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D80725