[analyzer] Move `TaintBugVisitor` from `GenericTaintChecker.cpp` to `BugReporterVisitors.h`.

Summary: `TaintBugVisitor` is a universal visitor, and many checkers rely on it, such as `ArrayBoundCheckerV2.cpp`, `DivZeroChecker.cpp` and `VLASizeChecker.cpp`. Moving `TaintBugVisitor` to `BugReporterVisitors.h` enables other checker can also track where `tainted` value came from.

Reviewers: NoQ, george.karpenkov, xazax.hun

Reviewed By: george.karpenkov

Subscribers: szepet, rnkovacs, a.sidorin, cfe-commits, MTC

Differential Revision: https://reviews.llvm.org/D45682

llvm-svn: 330596