[Analyzer] Bugfix for CheckerRegistry

`CheckerRegistry` registers a checker either if it is excplicitly
enabled or it is a dependency of an explicitly enabled checker and is
not explicitly disabled. In both cases it is also important that the
checker should be registered (`shoudRegister`//XXX//`()` returns true).

Currently there is a bug here: if the dependenct checker is not
explicitly disabled it is registered regardless of whether it should
be registered. This patch fixes this bug.

Differential Revision: https://reviews.llvm.org/D75842