[analyzer] Reduced the unwanted correlations between checkers living inside MallocChecker.cpp

This fixes an issue pointed to by Jordan: if unix.Malloc and unix.MismatchedDeallocator are both on, then we end up still tracking leaks of memory allocated by new.
Moved the guards right before emitting the bug reports to unify and simplify the logic of handling of multiple checkers. Now all the checkers perform their checks regardless of if they were enabled, or not, and it is decided just before the emitting of the report, if it should be emitted. (idea from Anna).

Additional changes:
improved test coverage for checker correlations;
refactoring: BadDealloc -> MismatchedDealloc