Still use BadArg bugtype in the checker. This saves us implement registerInitialVisitors

in the BugReport. 

When all internal bug checking logic are moved to checkers, BuiltinBug will
not reference GRExprEngine, and FlushReports() will be not necessary, since
all bugs are emitted into the equivalent classes immediately.

For now just add a ctor with no arguments.

llvm-svn: 80770